Here’s another terrific Ask-Design-Mom question:
Just thought I would throw this question out there and see if you have any fun ideas for homemade Valentine exchange cards for kids. I have three girls 10, 8 and 4. Thanks, Nicole Carpenter

Design Mom answer:
Wonderful question, Nicole! I had such a good time doing a little research for this answer. I went into my old magazine archives and picked out all the Martha February issues. What a treat to reread and remember so many great articles. I’m suddenly feeling all into Valentine’s Day.

I found about one bazillion ideas, and a particularly helpful article to answer your question in a 2004 Martha Stewart Kids. I found a link to a shorter version of the text here, but no pics, so I scanned images of the 3 best ideas from the article:

Flower Lollipops. A perfect project for a 4 year old.

Hand-y Valentines. I love the stamp lettering on these.

Dot Hearts

I also found 3 other ideas that might be fun for your older girls:

Sewn Glassine Hearts
I could take or leave the envelopes, but that heart is awesome.

Doily Envelopes from a 1998 Martha
The simplicity of this is smart.

You can also find directions for the Heart Bookmarks here. (From the picture at the top of the post.)
Would be cute clipped to a card, so the recipient could see it was a bookmark and not just a little heart.

Best of luck, Nicole! I think I’m going to try one or two of these ideas with my own kids.