The Animated Woman's super clever Lil' Valentines! She has both a color and a black and white (so your kids can color them in) version on her website.
Adorable woodland Valentines with printable envelopes by Inktreeprintables.
The Stir has a fantastic collection of even more free valentines cards and cupcake decorations to check out. My favorite Stir pick are these gorgeous grownup love notes by Amy Rice