In something like 98% of cases of child abuse the perpetrator is known to the child. So statistically the airline should exercise their duty of care by sitting children next to strangers.
I think the issue here has nothing to do with child safety. The practical reality is I do not want to be seated next to someone else's child if I can avoid it, particularly if the child is not supervised. So I expect the airline, in every ones interest, to seat children with their parents.
Maybe Ryanair should offer the option to pay not to sit next to an unsupervised child? I'd pay a lot more than £2!