Some great advice here. Communication with the parents is imperative. Let them know that you will not tolerate this, and that perhaps they should wait outside or in the waiting area during the lessons in case they end early due to tantrums. Let the children know that you will send them home if they pound on your piano, no more warnings. Tell them that your instrument is expensive, and that it is not something akin to a punching bag. Perhaps set a limit, such as if they have to be removed from lessons early due to pounding a total of 3 times, then they will be dropped from your studio so that they understand the seriousness of the situation.
If they get frustrated, give them something else to do instead, preferably non violent, like have a stress ball handy they can squeeze or a soft stuffed animal they can pet. Also you can try to use diversionary tactics when you notice their frustration (have some funny music jokes handy to tell them, or some funny pictures to show them). Getting them to laugh at their mistakes and hardships is a good way of showing them how to deal with things that don't work perfectly.
Frustration is expecting one outcome and getting another, and so if they do not expect to do it perfectly the first, 5th or 10th time they will be less likely to get frustrated. Sometimes students get frustrated playing on an unfamiliar instrument, and so I let those that have a hard time with this play through the song once and I tell them I won't listen. Then they play it again for me to listen to.
This is a great opportunity to help these children in life, not just lessons. Some might say that's not your responsibility, but you are responsible for the time you have with each child, which may involve discipline (not physical of course) and teaching them how to handle things in a mature way. If you don't teach them, then who will? Better that they learn this now than later when their job or marriage might be on the line.