We actually did this for my parents for Valentine's Day last year just before B was born. I think it turned out pretty cute! Aside from the fact that it's not arranged very well in this pic and C's hands look interesting. He was not even two yet, what do you expect?
large popsicle sticks (you can buy them already painted green)
craft paint in a variety of colors (including green if you need to paint the sticks)
decorative marbles or rocks (you can find these at the dollar store)
Coat each child's hands in a different color of paint and have them place their hands on a piece of cardstock. Let it dry, then cut it out. Write child's name and age on the back of one handprint. Glue or tape the "flower" onto the popsicle stick.
Fill vase with decorative rocks and arrange "flowers" in the vase.
It doesn't get any simpler!
Depending on how many kids and the size of vase, you could have each child do just one "flower" or as many as needed.
Grandma will LOVE it!!