It's not exactly small, like most new phones, but it's relatively heavy and thanks to the silicone cover that comes with it, it doesn't slip out of children's hands so easily. The battery easily lasts several days, which doesn't necessarily mean that it is used "less" ;-)
In fact, it is rather practical that the phones are too big for the trouser pocket, so the kids don't even get the idea of using the phones as a round-the-clock companion.
The phones are meant to be carried in the school bag or backpack, so that they are ready to hand when needed, and not as a constant consumption and possession machine.
This all works out very well.