No, not normal. They could be overtired, overstimulated or just unsure of things and testing boundaries - but certainly at 8 years old I would expect a little more curtesy and respect.

As for the violence - that is really not on. I know children can get excited and hit/kick - but this seems just nastiness, from what you say. I'd be putting them outside the house (physically, if I had to) and not letting them in until they apologise, but then I do not tolerate rudeness or deliberate violence.