Kids enjoy stringing beads during a wide range of ages, especially girls who might actually wear any necklaces they make. There are beads appropriate to all ages, although if you have multiple kids you might find the younger ones eyeing the older kids’ supplies pretty hard.
Have a good place set up for the beads when they come out. We store ours in a large plastic container with a lid, so it doesn’t spill when put away. Spills are enough of a problem when they’re being used, I don’t need to clean beads up when the kids want to use them. You don’t want the beads rolling all over the floor or sprinkled all over the carpet.
I tie a bead to one end of the string for younger children. The holes on beads for younger kids are often too big for a knot in the string to do the job.
Chenille stems also work well for beading. The beads don’t slide around as much as they do on string, so it can be a bit easier for kids to manage.
For older kids, consider introducing them to wire bending and beading. This is only for kids old enough to safely manage the tools, but it’s a lot of fun.