Because it's so annoying, the racket of keys struck at random may rattle your nerves, but it probably won't damage the piano. Tell them to use their fingers, then they should be able to play as hard as they want.
Most pianos are built to withstand very heavy use. The next time you see a serious pianist perform a flamboyant classical piece, notice how forcefully he or she attacks the keyboard. In comparison, a child's small hands couldn't possibly play that hard.
The real danger of children playing with, as opposed to playing, a piano is that they often can't resist dropping small toys inside, slipping coins into the slots between the keys, or running toys across the finish. Also, they should not pull sideways on the keys, or try to do Jerry Lee Lewis slides. The sharps are especially prone to breaking if they are forced sideways.
But, remember that music exists to give pleasure. Encourage your child to have fun with the piano, not to be afraid of it. Don't worry if young children play haphazardly and loudly. If you teach respect for the instrument and they discover how enjoyable playing can be, they'll treat it properly. And if your children learn that playing the piano is fun, you won't have to plead with them to practice when they're older.
And if they continue to misbehave, give me a call and I'll be glad to ball them out over the phone.