What bag suitable for children in primary school?

During my childhood I never use stroller bag, I am only using backpack and I have to hand carry another bag too.

My son is lucky today he gets to use stroller bag, not just him I saw many in his school boys or girls they are using stroller bag. It's easier for them less heavy to move around, unless you need to carry up the class. My mom in law says that son does not need stroller bag soon, it's better for him to use backpack. In my mind, it's better he keeps using the stroller bag, I don't want him to have any back pain carry the heavy bag.

I saw children carrying the backpack, the bag so heavy that I can see like they having hard time carrying it.

Perhaps my son can use backpack then he will need another handbag, but sad to say he is careless I scare he'll lost the hand carry bag. :(