Here are some other flower suggestions for children's gardens:
Gomphrena - From a small child's vantage point, the cultivar 'Fireworks' looks like pink puffballs reaching toward the clouds - how cool is that?!
Spilanthes - Also known as "eyeball plant," a cultivar like 'Peek-A-Boo' makes it easy to engage kids - they're not likely to say no to going outside to pick some eyeballs!
Celosia - Commonly known as Cockscomb, the crested-type like 'Cramers Burgundy' is easy growing and good for cutting. Plus, they have the unique characteristic of looking like fuzzy brains.
Whatever activity you choose, have fun and enjoy working in the dirt together - you'll be creating memories and perhaps even a life-long gardener in your child or grandchild!