With a six-year-old I would always choose the TATL day flight (AA 142) to London --- and after an overnight in a hotel near Heathrow airport, proceed the next day onward. It generally helps that 142 is also one of the easiest flights to find TATL redemptions on. Given how short a flight it is (often just 5 or so hours in the air, many times not enough time to watch a second movie), it frankly doesn't matter much what they serve or when.

[Without knowing exactly where you're going, it might be worth keeping in mind that BA has a new daily nonstop from London Heathrow to Zagreb starting Dec. 9, departing LHR at 8:25 am. Alternatively you can choose the LHR - VIE - BEG routing on Air Berlin/Niki.]