Be sure your hands have been thoroughly washed and dried so no bacteria from your hands contaminates the petri dish. (Unless you are following along with the YouTube video and swabbing your hands before and after you wash them).
Take a cotton swab, which should be in it's own sterile packaging, and take it out by the handle not the swab part. Swab the surface you have chosen.
Then take the same swab and lightly swab it over the agar solution at the bottom of the petri dish. Don't disturb the agar solution, just lightly swab over the top.
Seal the petri dish with tape and place a label on the bottom of the dish. Label each dish either with the child's name or what was swabbed...or both.
Place in a location in your home that isn't in direct contact with sunlight, but is in a warm place. Not too warm and not too cold, I just left ours on the countertop away from sunlight and not close to any a/c vents. Some petri dishes will show growth quickly, within a few days. Others may take a couple of weeks. You could have your children document along the way the changes they observe. I had my older boys write a small paragraph of what they saw with a drawing of the petri dish.