The CDC recommends that children sing "Happy Birthday" twice, to reach the suggested 20 seconds.
Other fun tips:
To teach children about germs and the importance of washing their hands, try the glitter hands game. Sprinkle glitter into the hands of two or three children. Use a different color glitter for each child. The glitter represents germs. Have the kids shake hands and examine their own afterwards to see how the glitter (germs) spread from one person to another throughout the day. After, show the children the proper way to wash off all of the glitter, and instruct them to do the same every time they wash their hands.
Create a hand-washing chart to remind kids to wash their hands throughout the day. The kids can earn stickers when they remember to wash their hands without a reminder.