Read up on Business Link employing young people, it explains the legal side. The judgement for what is a dangerous work activity is reasonably subjective but given instruction, then if cleaning toilets/bathrooms are still considered too dangerous to clean for them, then they must be different types to the ones I go in. People get confused with regulations- misinterpreting the idea behind them.
Personally I would not employ a 15 yr old cos they tend to be sh!te cleaners, but of course there are exceptions to my sweeping generalisations.