Here's an idea that can come in handy if your child has lots of people on her valentine list -- photocopied prints she can have fun custom-decorating for each recipient.
On plain white paper, trace your child's hand in pencil. Draw on heart-shaped fingernails. Retrace all the lines with the marker and add a Valentine's message, such as "I want to hold your hand."
Have the design photocopied. Then your child can use markers or crayons, glitter glue, and stickers to paint the nails and add glitzy jewelry.