Flying overnight with a nuts and hyper preschooler could work out either ideally (if he sleeps) or truly dreadfully (if he doesn't). Flying during the day will be predictably miserable, especially if you leave on an earlier flight.

If you have the time and are risk averse, I would consider breaking up your return (eastbound) flight: Daytime flight to the east coast (e.g., NYC), a night in hotel, and one of the few daytime eastbound transatlantic flights to London.