I'm sorry you haven't gotten any answers. I really don't know either.
When I had gliders over groom the hair off their tails, it was due to stress. One was lonely and stopped as soon as I got her some cagemates. The other was stressed by being near a colony that rejected her, so I moved her to another room.
I have a boy who currently overgrooms above his eyes on his forehead. I haven't been able to figure out why and it varies between pretty bald to slightly bald. At this point I think it's just his habit now so as long as it doesn't get worse, I try not to worry.
So, maybe if you write a little more info someone will be able to offer more advice.
Here's some clarifying questions:
what size are the cages?
How close are the cages?
Were they ever in the same cage?
What are you doing to help them bond?
Are you scent swapping?
What toys do they have to play with?
What diet are they on?
Did your 5 year old ever have cagemates before?