I'd want my kids to be exposed to CP as it's much easier to deal with a child having it than going your whole childhood not having it and end up with the possibility of shingles as an adult, which is much worse and dangerous.
The older girl seemed to already show signs of CP which means she was not contagious. Yes, her brother might have been, but no point in keeping him home until he gets it, and then once he gets it as well.
welcome to life at daycare, unfortunately!