It’s hard to say if it would be too much not knowing how the kids are. However, based on the ages of the children, it should be fine and the 8, 9, 11 and 12 year olds should be able to entertain themselves and capable of behaving and even helping out with the younger ones if needed!!
I would actually have out some games, snacks and a movie prepared for everyone to have a good time and hangout. It really shouldn’t be that chaotic.