make sure you find the new children's play areas they´ve installed. They're free!
They´re small, but have enough to keep little ones amused for a while. But they're only on a 10 week pilot scheme till September, so make sure you leave a positive comment, so that it'll be carried on.
The staff said they were in all terminals, and the one in T5 is the biggest. I was in T3, and it's a bit hidden, on the way to gates 23-43 if I remember correctly, but only actually a minute's walk from the cafes and shops.