I was also concerned about breaking keys or action parts by striking
the key very hard when tuning.  Ed McMorrow who tunes my piano assured
me that there is no risk other than breaking something that needs
attention anyway.

A proper piano action is built in such a way that it can take pretty
much anything you can do at the key end and not cause damage unless
it is in bad condition or horribly regulated and if that is the case,
there is little lost and you would probably be wasting your time tuning
it anyway.

This also suggests that it is perfectly safe to let kids pound on the
keys.  It's good to know that they do not need to be stifled in their
expression.

Best regards,
Spencer Chase
Garberville, California
http://www.spencerserolls.com/