I'd say it depends on the time of day Bleary, in 8 hours they would likely have to account for one of the rush hours if not both and since I am going to guess they are going to be there during the day, since they are looking at heading into London with small children, they will be travelling with the rush hour not against it.
At a minimum you are talking 2 hours out of your 8 spent travelling (I'd leave at least three for comfort). As you will have to check in 2 hours before the flight and, if you are flying Ryanair, you can't check in until 2 hours before your flight, this means you need to plan to be back at Stansted maybe an 1 1/2 hours before so it becomes 3 hours out of 6 1/2 hours.... there are cheaper, easier, closer ways of entertaining the kids for a few hours. I'd opt for one of those.