Say it's a public changing room of a swimming pool. From what age should a father stop bringing his daughter into males room with him? (Is it different for mother-son?)
I know it's probably very culturally dependent, so please say where you are. I live in the UK btw. On the other hand, is there any law regarding this (in any part of the world)?
I don't know if there are really laws for that, but there are laws for harassment, that give an upper limit to taking your kids into a changing room. Usually laws of western countries define children as people that between 12 or 14 and younger, as long as your kids are children by law, they can't harass other people, and you should be save from this perspective. But there are social limits, I personally wouldn't take children over 6 years with you in the "wrong" changing room. For once children are likely to make fun of other children that are in the "wrong" changing room. And your own kids will someday start to feel bad about being in the wrong room.
And yes women will get away longer with taking their sons in their changing cabins that men taking their daughters with them.
I am currently in Texas and here, there are family change rooms with cubicle doors at our local pools. There are three rooms -- men and boys, women and children under 5 and girls -- and family rooms -- men, women and children. There is usually a staff in there. Our pool's family room is attached to the lifeguard's office. Plenty of mums and dads stand outside the cubicle while their children change.
MY ex-hub had a problem when he and my daughter were travelling. Years ago, there were no family bathrooms in that airport and he could not take an 8 y/o into the men's room and nor could he enter the ladies. He found an airport employee with a badge to go in with her. I imagine that situations like that encouraged 'family restrooms' in public places.
While I personally see nothing wrong with families seeing each other naked, I also think children get modest and when they indicate that they are uncomfortable, then it is certainly time. My 16 y/o has seen her step dad naked once or twice when she walked into our bedroom. He just turned away and continued dressing. (In other words, no big deal, but knock next time.) Her father stopped seeing our daughter naked around six years old, when she told him to knock.
As a practical addition to the other answers:
At least in Germany, many public swimming pools have a solution addressing this problem: family changing rooms or family locker rooms (Familienumkleide). These are mixed-gender changing rooms which are specifically intended to allow families (or a parent with a child) to change together. In Germany this is typically a single large room shared by all families needing one; sometimes and in some countries there may also be multiple family changing rooms for a single family each, instead of one large room.
A casual web search shows this exists in other countries as well, so you might check if a swimming pool has one. I have used them together with my small daughter, and found them a very practical solution, easier than trying to cram two or even more people into a changing room intended for only one person.
If a facility does not offer family changing rooms, they may have single-person changing rooms (basically many small cubicles). In a pinch, you can probably use them with two persons, so father and daughter or mother and son could share one. This is usually not practical with more than one child, though.
