I know this probably seems silly, but my boys outgrew the cheap children's hangers, and their clothes were always slipping off the hanger.
I bought a case of 100 of these for $21.45 plus shipping (mine was $9.75 for UPS ground shipping). They were speedy in shipping too. I know it's more expensive than the Dollar Tree, but I finally took the plunge and fixed the problem! No more frustration.
Junior Dress Hangers
They fit my boys who wear 7x to 8 and will definitely fit through more years.