I don't see any issues, so long as they are reasonably sensible and mature. The system treats them as young adults, which is what they are at that age. Both LHR and BOS are well laid out, T5 particularly so. They will almost certainly be departing from T5B, so that's the only slight problem. You could show them the passageway guide in the Forum Dashboard if you like, just because it's got loads of photos, but I guess the transit is easier. You could book a domestic flight so that you can get them to the departure gate in T5, but frankly it's not necessary. So long as they charge their phones up, and you are on standby to help if they find themselves in an unsure situation, and get to the departure gate an hour from departure time, everything else is difficult to get wrong. At BOS there isn't much optionality, there is in essence only one way out. I'm sure they will think it a great adventure, and so long as there are no personal complexities, I would just let them get on with it, they will never, ever forget that flight.

I would keep it simple for their first flights, so I'd avoid transit in DUB, though that isn't so complicated really.