We needed a very easy solution to making about 40 valentines - our children are 5 and 4. I bought 2 poster boards, put out the plastic tablecloths, and let the kids paint, glitter, and decorate the poster boards. Then I cut 20 hearts out of each. Much easier than cutting the valentines first and then letting the children decorate each one. Its not overwhelming for the littler ones.

Poster board.
Paint.
Glitter.
Other things to decorate, i.e. doilies.

Let the kids have fun!