[Lost Dog]

If they aren't strong enough to do a pushup, then start them doing pushups at an angle. Hands on a bench or step risers.

I like the risers, because you can start lowering it, week by week, after they hit about 10-15 at that height.

The higher you go, the less weight on the hands, shoulders, chest, and core. So, you train the whole system to get strong enough at the same time. On the knees won't cut it. It's not the same movement.

Another thing. Newbies don't know to make their body rigid for pushups. Make sure that you train that or they'll sag all over, and get fewer reps. My ten year old went from 5 to 10 reps by stiffening up and using his whole body to do pushups. In the last few, even your toes, calves, and ass are helping to push you back up.