Have similar issue....a 4th grader who is very very developed has almost full breasts and they are quite large and she forgets to wear a bra...some days ill find her not wearing one she simply forgets. shes just so little emotionally. Shes 9.
Too young to need a bra but she cant walk around without one ! When I tell her to go put one on she says whatever its fine I dont need it!
Get her a tank top with a shelf bra
That should solve it.