I don't see the big deal, OP. While I have no problem with small children running around in just pants, or wearing age-appropriate bikinis for that matter, I can entirely understand why you were asked to have her wear a top in a holiday club.
Plenty of places have rules about appropriate attire - there are hotels with rules about no swim wear in certain areas etc - and it is simply not practical to expect staff to exercise their discretion on each and every case, given the different rates at which children develop and the difficulty in estimating a child's age accurately. Clearly, at some stage in their development, it will become appropriate for a girl to cover her chest in most social situations. The boys v girls argument isn't really relevant as we live in a society where women's breasts are seen as private, and men's chests are not - that isn't going to change any time soon. So at some stage, every girl will find that it is no longer appropriate for her to be topless.
While it should be the case that parents can be left to make this decision, the fact is that whenever there is a rule, there will be people who don't want to obey the rule. A holiday club can't have its staff constantly embroiled in arguments with parents about whether the rule should apply to their individual child. There is also huge potential for embarrassment to the children themselves. Can you imagine two girls, one of whom has begun to develop breasts and is self-conscious about it, the other who has not started developing at all...and is self-conscious about it. The staff make a judgement call that the older-looking girl must cover up but the other one need not - an argument with the parents ensues - "oh but they are both the same age". Two very embarrassed children are the end result.
Is it not so much more straightforward if there is a blanket rule so that no child has to be singled out as "too developed" - they are just told "look, this is the rule and it applies to everyone."?