M.P.
Snacks or lunch.... pick up chicken nuggets from Chick fil A... they sell it by the tray, you can even order in their website and just pick it up and in some cases they deliver. http://www.chick-fil-a.com/restlocator/orderfood.html
If that is out of budget you can make simple ham, cheese and mayo sandwiches and cut them in hearts.
You can acompany that with either fruit that you buy at the grocery store like grapes, already washed strawberries and bananas or at chicken fil a they sell also a big bowl/ tray or fruit.
Bring individual juices. one per kid.
As party favors.... you can either buy a big chocolate in hear shape, do little bags with filled with candies or a big lollipop with Valentines motif.
The home made touch, make cupcakes from the pillsbury box, with white icing and little red or heart shaped sprinklers that right niow you can find absolutely everywhere.
As Crafts.... you can go visit familyfun.com they have a lot of ideas there or stop by any Michaels and they craft kits, They have a good selection.
You can have the kids make a heart shaped card glue some little somethings, including glitter, put their names for their parents.
Hope this helps.