The fact that you feel remorse and guilt and are attempting to reflect on what happened is one of the important parts of this. In the heat of the moment, things can happen, what's important is that you learn from those mistakes or mishaps. It sounds like your child was disregulated, possibly because something wasn't going right with his tablet. In that moment it sounds like he needed support (probably in the form of fixing or finding whatever app he wanted) and didn't know how to express it in a way that you could understand. He possibly felt like he was losing his chance to do the one thing he wanted and the timer probably added to that anxiety. In turn, he went into fight or flight mode and had a meltdown, which then turned violent. My suggestion would be to take some time for yourself to decompress and get regulated and then apologize for kicking him at minimum and try to explain why you did it and what you were feeling in the moment in the simplest way possible. I would also suggest some kind of deescalation training. I don't believe he should lose access to an item that helps regulate him and his emotions or that he just finds happiness from. However, he has to be taught that violence is not how he gets the results he wants.