Joh "Voluntary" is not enough, the parents must not hold the key at all. Which admittedly gets challenging to prove one way or the other, if the kid was found at a school or hospital to be wearing a chastity belt, the parents might just give her access to the key in secret. Or she has the key in a sealed envelope, but the kid is made to lie that she is proving her non-use of her key to a friend instead of her parents. Or what about church groups that set up a peer-based keyholding system, but with enough guilt tripping that someone will blab to a bad actor if they released themselves for non-church-approved activities, and with a strong expectation that girls join it. Basically a stronger purity ring.
Of course one could make up endless scenarios where abuse would be possible, but even beyond sexual experimentation, there are also valid reasons for a young person to wear one. Maybe a friend has been attacked recently and she prefers to carry protection when she goes out. Or maybe she is afraid of mistakes she will make if she gets excited at the club. You can sacrifice a lot of individual liberty to the temple of safety, and the question where to draw the line will always be a matter of debate. Overbearing parents can abuse their children in lots of ways with or without chastity belts, and some children are also going to be sexually abused in a most straightforward manner by immediate family members. There is no 100% safety from abuse, which also makes it hard to draw an objectively true line.
My line of thinking is that by making it difficult to get a custom belt fit, you are limiting the belts minors can experiment with to off-the-shelf items, which at least as far as full belts (and thus pretty much all viable options for girls) are concerned are not very suitable for 24/7 wear. Maybe a society could also have other support systems in place, like agencies that can demand to keep copies of keys to devices a minor wears and enable them to take them off in a way that parents do not find out about. Maybe regular use of their services to monitor the consensuality of their use would be the "price", so to speak, of wearing a chastity belt as a minor.
But then again, maybe those agencies have their own abuse potential, with agents having a steady stream of rather undressed girls to take care of. Now that I think about it, the very thought of that is making me uncomfortable, so maybe you are right at least as far as banning underage device use completely being the easiest way to limit non-consensual use on minors. Frankly, the thought of coming up with more alternative options is kind of making me sick right now.
None of that solves the issue of forcing chastity onto adult dependents, though.
