I can see the mother's point with the nail polish. I don't think I'd let a 6 year old loose with a glitter polish set without supervision - nail polish is a real pain if it gets on clothing, furniture, or the carpet. So the 6 year old loves the gift, but I have to be there, watching and helping, while she uses it - which might not be all that convenient for me, even if I like the kid.
The way the mother handled it was inappropriate though. She should have said thank-you, and at a later point mentioned that the kids are still too young for messy/craft type stuff without supervision.
One thing that occurs to me - is it possible to to give a gift that like for use at *your* place (ie, you live close enough). Then you can have the kids over for an afternoon, and do nail painting, or sand crafting, or paper dolls, or whatever, and you can deal with the mess and the supervision. That way the parents don't have to deal with the fuss, and you'll get a better idea of what these kids in particular can manage.