It's a normal phase but it's annoying. If it plays itself out, great. If not, then try not to overreact but also try to channel it. I agree that talk about bathroom stuff belongs in 2 places: the bathroom, and the pediatrician's office. So send them to the bathroom for that talk. If it goes on in the car, tell them you don't want to listen to it. Don't be horrified, but be bored. Tell them to stop. If they don't, pull over and get out of the car. Have a book ready or a crossword puzzle or play a game on your phone, anything. Remove yourself from the car, stand where they can see you but you cannot hear them. Turn the car off unless it's incredibly hot, in which case obviously leave the AC on. Stay out of earshot for 5 minutes and tell them they have 5 minutes to finish potty talk. Be clear that you are occupied doing something else and not that you are steaming mad and that they got your attention - instead, put your attention on something else.
After 5 minutes, get back in the car and start on your way again. If they start again, pull over again. Really. Be prepared to not get to your destination on time or at all. That's okay. You have the right to peace and safety in the car, and letting them do endless potty talk just because they aren't fighting is still giving them the power to determine the rules. We always taught our son that it's not "funny" unless everyone is having fun. That also stops bullying behavior, when kids make others the butt of their jokes just because a few kids think it's funny.
At home, send them to the bathroom or their own rooms to talk that way. Be bored. You have the right to be at peace in your home too, and decent manners.
If they do this at play groups or social situations, take them home. Let them know that other people don't find it fun, that it's bad manners, etc. Again, let them talk that way to themselves but not to others.
If you are consistent in not being part of their audience, they'll stop.