Skip to my lou has an adorable way to decorate the kids' bedroom on Valentine's day by placing several paper heart messages on their doors. You could also decorate all around their room or bathroom mirror.
Artful kids has a great way for the toddling set to make their own valentines. Have the kids finger paint onto a sheet of paper. Then glue that paper to a card with cutout hearts on the front. The kids' fingerpainting will show through the cut outs to make a very pretty card,
Chez beeper bebe has a great valentine for the lego fan. She creates a valentine and attaches lego pieces in a clear bag to the back for kids to construct their own ring.