Purchase polymer clay from an arts and crafts store in a package large enough to accommodate both of your child's feet.
Knead the clay with your hands until it becomes warm and soft. When it comes out of the package it's generally stiff and cold, making it hard to get an accurate footprint.
Mold the clay into the shape you want the footprint to be set in, such as a circle, oval, square or star.
Press the foot or feet firmly into the clay to get an accurate impression. If you don't get it right the first time, roll the clay back up and re-form it before making another imprint.
Write the date, name or any other information beneath the footprint in the clay using a toothpick. If you want to use the prints for an ornament, poke a hole near the top with a straw.
Put the clay on a cookie sheet and bake at 250 degrees Fahrenheit for 30 minutes.