One problem I've had in accomplishing crafty projects with my 3-year-old is lack of preparations. Trying to do projects on the fly tests both our patience. For making Valentine's Day cards this year, I found that a little preparation went a long way in giving her hours of fun.
Gather your materials and set up a work space. A child-size table or coffee table works great, but so does the floor.
Cut out 10 large hearts (or as many as you need) from variously colored construction paper.
Cut out colorful shapes in various sizes using additional sheets of construction paper and/or colored felt. If your junior friend can handle a safety scissors, this is a good step to involve her in.
Open up the paste pot and stand back. It won't take long to see the creativity flowing.
Be available to give advice and assistance, but don't overdo it. Your junior friend may need help with spelling and pasting thicker items (like buttons and popsicle sticks).
The most important grownup role is in helping to stop one Valentine and start a new one. When one card reaches a critical mass, it may be appropriate to suggest the name of another friend. Present your young friend with a fresh heart. Put the finished card out of sight to dry. (You can also quietly make any necessary fixes.)
Clean up and enjoy. Happy Valentine's Day!