Types of paint safe for children to make handprints out of, on canvas
If they wear clothes you don't mind ruining (good quality acrylic paint will not wash out of cloth completely, ever) and you supervise and help them wash off the paint afterward and put down a large drop cloth to protect carpet/wooden floors (tile is cleanable), or do it outside, student grade acrylics should be fine.
At those ages they are unlikely to try eating the paint.
I did something similar with my daughter and nephews as a gift for their grandparents when the youngest was only three (granted a reasonably intelligent 3) only with many different prints in various colours.
Make sure the canvas is primed for acrylic. You can just buy a pre-stretched primed student canvas at an art/craft store or order online in the size you want. That way you don't have to have it framed.