Start off with wall push up. Get your child to stand in front of a wall and place their hands on the wall at shoulder height. Your child should then bend their elbows keeping their back straight. Then push back to the starting position. Try to complete 10 wall pushes.
When your child has mastered wall push ups they can move onto floor push ups. It is easier to do push ups in four point kneeling to start with and then to progress to straight legs. Make sure your child pushes up with straight arms and keeps their back straight.