I would NEVER give riding lessons to a 3-yr old! The parents are living out some fantasy through this poor child.
And, you are NOT a babysitter, so why try to teach her ground games? be honest with the parents and tell them she is just not ready emotionally nor physically.
Or, you can always keep taking their money until she turns 5! (kiddin)
As a trainer, I never stop learning. Just as a horse learns every time you ride.