In the event of disruption the railway has a responsibility to provide a means for a passenger to reach their destination. Potentially a different train company, alternative route, or a replacement bus if the railway is completely out of action.
In the worst case of there being no other option to replace the last service of the day, the railway may provide a taxi to the final destination or, as a last resort, overnight accommodation. In those circumstances, what happens to under 18s - no hotel will take them, and if taxi is an option they are likely to refuse unaccompanied minors on the basis of child protection.
Are they given over to the care of the police? I can't think of another option!