1. Even if we are, for example, only talking about the child of a club member, there still needs to be a policy of some kind. There is always the chance that a minor will be simply taken on board as "one of the group" - i.e. any possible vulnerabilities they have as a developing person may be disregarded by other cavers present. I suppose it would be the parent's responsibility to protect their child in such circumstances - for example, removing them from any "adult" behaviour they may not be comfortable with.
2. PoCA clearly spells out "abuse" as including "sexual", "emotional" and "physical" abuse and also "neglect". Neglect in terms of caving must be taken to include failing to ensure physical safety, adequate/appropriate nutrition and protection against hypothermia, drowning, rock-fall, injury through falling, etc.
So taking a minor underground, especially where the trip might be something of a physical challenge for the minor concerned, carries protection responsibilities beyond those of sexual abuse, which is where the focus seems to go most of the time when talking of abuse. How often have you been in a situation where you have consented to letting your son/daughter go underground in one group, where you might have opted to do something else? This may not be a problem - your club's policy should provide a means for this to be possible. This is a bit off the subject of accommodation, but has a relevance to the overall picture.