You know that in many places in the continent, topless sunbathing is considered family friendly. It's literally why you're having this conversation.
If you think that nude boobs are so dangerous for your children that you don't want to risk them seeing any, then you need to contact the hotel to clarify whether it's allowed there and if it is, you need to find somewhere where it isn't.
Seems a very small trouble when safeguarding your children, no?
You don't get to go somewhere where it's the norm, complain that it's happening and use your children and the notion of "consideration" to try to demand that the acceptable, established norm in the place you are visiting changes for you.
If you aren't prepared to do a little inquiring and research for the benefit of your children, then that suggests they are not actually the real reason for your disapproval. And if they are, it's hard to see why you wouldn't take sensible measures to protect them. Unless you don't actually think there's any harm in it, but you don't want to admit that the issue is with you.
Don't like it, go somewhere where it isn't an accepted norm. You still haven't said why this isn't an option.