May sound strange, but it's a fab day for toddlers/small children. We took our 2 1/2 year old and 10 week old the other day. Terminal 1 has a fab viewing lounge and although there are loads of restaurants there we took a packed lunch so the day was free - except for the train fare.
So, fun time was had by all - lots of planes/trucks to see and a train ride at either end!