I have to agree that clipping a 3 month-old baby Macaw is a terrible idea, because it's actually the opposite of safe. It's not what is best for the bird in any way, nor is it what is best for your kids. I know you think that it is, but that is actually counterintuitivve thinking, and a common misconception...
And we're not talking about the fact that the Macaw has already fully-Fledged...No, you cannot ever clip any baby parrot before they fully Fledge, because that results in terrible and permanent Neurological and Behavioral issues...But that has nothing to do with why clipping this baby Macaw, who has either just fully-weaned or isn't yet weaned and that you're going to have to finish hand-feeding (can't tell what the situation is there, you shouldn't bring him home until he's eating 100% solid food and you don't have to give any formula feedings at all)...Also, just a point, your friend who is the breeder, and who hopefully is not giving you an unweaned baby bird, also shouldn't be allowing her breeding-pair to breed again that quickly...It is not at all a good thing that "The parents have already laid 3 more fertile eggs"...That's not something to brag about, it's very unhealthy for them and can result in not only dead babies, but also a dead mother...But that's another issue that isn't your fault...Just make sure that your new baby Macaw is 100% fully-weaned and no longer needs any formula feedings before you bring him home...
***Now, as far as "grounding" your Macaw...What typically happens in exactly your situation where you have kids or adults who are scared of ALL birds, even very small parrots, so the owner decides to keep them clipped "for the bird's safety and the kid's safety", is that the new baby actually ends-up becoming extremely skittish and scared overall (regardless of the fact they were hand-raised), because they do not possess the ability to get away, and this results in an extremely aggressive bird. And that's probably how this is going to end up for you if you clip him at only 3 months old. Then there are the serious health issues that can result from clipping a baby bird so young, the most common being muscle atrophy, a total lack of "muscle-memory" developing, so that if you decide to let their feathers grow back in at some point in the future the bird actually never flies again because their muscles/body has forgotten how, simply because they were clipped so very young...And this is EXTREMELY COMMON in the larger parrots, specifically the Macaws and the larger Amazons...
***I totally agree that the answer to your problem isn't at all to clip your baby Macaw, but rather to expose your kids to a loving, friendly baby Macaw that can fly, so that your kids can understand what a loving, friendly relationship with a parrot actually can be...And how do you accomplish this? Well, you said it yourself: By having your kids, spcifically the 13 year-old, participate in actually Recall-Training your baby Bird from day #1...But you absolutely cannot Recall-Train a parrot who is clipped and cannot fly!!! Your kids not only need to be educated about parrots and their normal, common behaviors, personalities, intelligence, and their ability to be trained to do just about anything. But if you clip your 3-month old baby Macaw's wings out of "safety concernes", all that you're teaching your kids is that it's okay to put your bird's future health and well-being at risk in order to make them feel safer around the bird, which makes no sense at all.
Instead of clipping your Macaw at 3 months-old and not only putting his overall physical and pschological health at-risk, along with risking that he'll not be able to fly ever again in the future, but also taking away the trait that actually defines him as a bird, I would implore you to start doing both Target-Training and Recall-Training from the very first day you bring him home, and INVOLVE YOUR KIDS IN EVERY BIT OF THE TRAINING! This makes a lot more sense than clipping a 3 month-old baby Macaw, which will accomplish nothing at all...You don't even know if your Macaw is going to exhibit any behaviors that clipping his wings would help with, the main one being "dive-bombing" at people...But you have no idea if that will ever be an issue...So why in the world would you just have him clipped before you even get him home?
Something that is extremely important to consider as well, and that you probably aren't even aware of because you've only owned small parrots, is the fact that large parrots, specifically Macaws with their huge wingspan, don't even fly much inside at all anyway, because they know they don't have the room!!! They aren't at all like the smaller parrots who are able to fly all over the place and navigate around corners and such, they are typically on-foot most of the time anyway, with the bulk of their indoor-flying being during Recall-Training from perch to perch or person to person, which is extremely important for them to be able to do. So there is absolutely nothing that clipping his wings is going to accomplish...He can walk over and bite your kids on-foot much more quickly than he could fly to do it indoors due to his size and his wingspan...So instead of clipping his wings, which will only hurt him and not do ANYTHING at all to help him or your kids or anyone's safety, why not leave him fully-flighted so that he has the ability to participate in Recall-Training and other training exercises, and then INVOLVE YOUR KIDS IN HIS TRAINING, which will not only keep everyone safe, but is the best way for your kids to become educated about parrot behaviors (natural behaviors, not artificial behaviors that clipping a 3 months-old baby's wings will produce) and for them to also feel more and more safe and secure in handling birds over-time? There is nothing better to teach kids about natural parrot behaviors and psychology than teaching them to properly handle a flighted Macaw...And he is a hand-raised baby, so there is aa line of protection that this provides as well versus a parent-raised bird...
Just clipping your Macaw's wings before he even sets foot in your home is not going to help you at all, not going to help your kids progress in their parrot education or help them get over their fears, and it is only going to result in lots of future physical, psychological, neurological, and behavioral issues for your Macaw. And again, it's not going to do anything to make him or your kids safer, in-fact it will do the opposite because it will only cause your Macaw to be startled and afraid of everything himself, from properly balancing on perches, climbing things like his cage, stands/perches, and toys, and knowing that he can easily get away from any dangers, OR easily get to his other Flock-members (you and your kids) to protect them from any danger...And it also is going to severely limit you in the training you and your kids will be able to do, which is actually the absolutely best way to teach your kids to respect a flighted parrot, but at the same time not to fear them because they will know how to properly handle a flighted large parrot. Wing-clipping is not a cure for much, there are certain home situations that benefit from it, but this is not at all one of them. And the fact is that your Macaw is just far too young to take away his ability for fly, and I'm pretty sure that you'll be sorry that you did it in the future due to the damage it will do to both his physical and psychological health and well-being...