I guess high-quality plastic is the best you can do for children. But I am VERY concerned about plastic, even #7. It definitely leeches bisphenol-A.
I am at home a lot so I just drink when I'm at home, mostly.
I have a stainless steel and a plastic thermos, I alternate between the two if I need to take a drink with me.
Mason jars are good, I think I will start using those but I have to buy some new ones, I can't get rid of that pungent salty smell since I used them all to make fermented vegetables like kimchi! ha!