5.5yo boy and 2yo girl, not just bathing together, but regularly stripping to nothing to play. I didn't care when we lived in a built up area and I certainly don't care now that we're on a farm!
I love how my BiL's family are with nudity - 16yo still showers with the little kids present in the bathroom, 14yo sister bathes with the little ones, who are 3 and 5 and boy and girl. They've always been so normal and unrepentant about nudity