A pre-paid hotel stay means you pay when you book the room.
The problem is that some hotels also request a credit card (or debit card) on arrival for incidentals, such as use of the phone or minibar.
That said, a number of airport hotels seem to have the ability to lock these items out of action, as they frequently get big groups arriving off cancelled flights.
What I'd do is find a good pre-pay rate then before you book, get onto the hotel and ask what the situation is with this.
They may also settle for a cash deposit, but then you have to queue up in the morning to get it back and as they'll want GBP, you then have the headache of having to take that with you.
Edited to add - dunk has come up with a very good solution in my book.
You'll be too late for dinner anyway, and just get breakfast at terminal 5. Note you'll have to account for the hotel hoppa charge too, which will be £4 each way for you, and free for your son).
Last edited by Swiss Tony; Jul 17, 2012 at 9:17 am