We have the yumbox & use so young bags.
We fit a sandwhich cut up, jelly, fruit, dip, biscuits & a special snack.
We sometimes use a reusable sandwhich bag & use the other compartment for yoghurt etc..

my 6yr sometimes comes home with leftovers but my 7yo doesn't.
So I'll probably be getting a go green or planet box for her next year.