No matter how much of a punk the kid is, you can't say it's okay, EVER.
because the moment you give the cop the power to determine which 'smart mouth' should be shoved down, you have a bigger problem. The cop can now shove down 12, 10, 8, and 6 year olds who talk back. The cop can now shove down 60, 70, 80, and 90 year olds who talk back.
The moment you okay it for one incident (even if the kid is a douchebag) the minute you have a police state where ANYONE can be beaten at ANY TIME for ANYTHING.
This cop should have locked the kid up on whatever law he was breaking - disturbing the peace, etc. Spending a night in juvenile hall getting shoved around might keep the kid from coming back to that park.