I'm not an expert but all I can do is explain what I'd do in your situation. I hope it helps! Obviously my pair are eclectus and not conures but hopefully the method should transcend across species!
Each bird will have is own preference with regards to treats. My Bella will work for pomegranate arils, which is nutritious but messy OR slivers of almonds, and Dexter will currently only work for banana chips, (but I keep trying different things in an attempt to get him to convert to something healthier!)
Find His Favourite Treat
Just try offering Coco different things until Coco clearly loves something! Then that's what you'll need to start training with. So you'll need a bit of trial and error in finding what treat motivates Coco the most. Keep in mind, it needs to be something that doesn't take long to eat. And only small so you can give many.
Reward Ignoring The Child's Presence
So at first, have the child as far from the cage as possible, at this point you are rewarding Coco for giving no reaction for the child being in the room while they are far from the cage.
If Coco starts going loco in his cage from the child's presence, turn your back on Coco. He wants your attention and your treat. Once he starts to behave normally again (no aggression) reward him straight away with an affirming 'good boy'. Hopefully after doing this consistently, he will build the association between behaving well (ignoring the child) with treats and your attention.
Behaving aggressively = being ignored and no treats!
Start Target Training In The Cage
Once he's gotten used to treats coming from you - start getting him to work for them! Tap a perch with the added verbal 'come here'. Reward with a treat when he goes there and say 'good boy'. Initially start with a perch close to where he is already to try to make it easy to begin with, once he gets the jist of this, gradually tap perches that are further away from him.
Gradually Increase Proximity
Once he's fine with a child being in the same room at a distance - reduce the distance slightly (where he's still comfortable and not behaving aggressively). You can do this over a number of days, maybe only 1 step at a time (or maybe over weeks depending on how fast he learns).
Eventually you will reach the point where the child is close to the cage, and Coco isn't interested, as he's focused on waiting for you to show him and tell him where to go in the cage to get the treat.
Start Working With Cage Open
Use the target training for perches inside the cage, including a perch on the inside of his cage door (with the door open). Initially keep the child further from the cage (like before) and work towards bringing the child closer over time.
Start Working Outside Of The Cage Too
Start including locations outside of the cage (perch / playstand / table?) as well as inside the cage using the same method. Again you can initially have the child a bit further away to begin with and gradually Increase the proximity.
Eventually all of this should culminate into being used to the child being around without being phased while associating their presence with treats!
This process could take minutes, hours, days, weeks or even months for him to understand. But the most important aspect is being consistent. Always reward good behaviour and always work at a pace that your Coco is comfortable with.
Good luck!