With an umbrella, I personally would recommend limiting out of cage time WITH the child until the bird has had plenty of time to hear the child's voice and acclimate to them (like SailBoat said, slow movements, avoiding sudden noises etc). Is it possible to keep the child out of the room when your cockatoo is roaming? Umbrellas are more likely than a lot of other species to just take off and attack someone, and I'd hate to hear of your kid getting a nasty bite. Someone else might have a different suggestion, but that's at least my take.
A lot of animals aren't big fans of kids. It's a tough situation you're in, because it's also a big ask to expect a child to slow down and quiet down and have the patience to stay that way patiently until a bird comes around to them.
Edit, that said congrats for rescuing! Umbrellas are one of my favorite species of bird, even if they can be a bit intimidating and possessive. I'm counting the days until I can rescue one myself. Good luck, whatever you choose to do!