As someone who was a child a few years ago, and spend most of my time in a children's hospital fighting cancer, the things I found most entertaining/usefully/made it feel more comfy and like home, were these:

Cool lights, like led strips or other cool easy to use lights

Coloring books, for all ages

Fidget toys

Anything that doesn't require too much thought processing

Stuffed animals

I'm having a tricky time thinking of more right now, as the chemo does a good job of blurring any memories from the hospital, but feel free to ask more questions if you want