Kittenpie demonstrates that a) toddlers aren't too young to make Valentines, and b) "handmade" doesn't have to equal "fussy."
Last year, I cut up large-ish squares of thick paper, put them two at a time on the easel, and gave my daughter red and purple paint. She splotched them up good (she was 2), had a great time, and then I stamped a rose and a simple to and from message on the back. Handmade, valentines-y, and very simple. But people loved them because they were obviously painted by her!
An even simpler hack would be to take some of the kid art lying around the house (perhaps the stuff destined for the recycling bin) and use THAT as the basis for heart-shaped cards. Same handmade warm fuzzies, less clutter and work!