Heathrow airport is a hub so having a layover there is fairly common. We were thrilled to find out about their kids area. It's hidden, but if you ask and follow the signs down back hallways, you will eventually get there. When you enter, you check in letting the person working know your outbound flight. She (the person working when we were there was female) will make sure to let you know in plenty of time so you won't miss your flight.
Our kids immediately went to explore the playground. I couldn't take photos of anyone other than our kids so I can't show you everything. There was a medium sized playground that our kids were playing on. There was also a small crawling area for smaller babies and toddlers. Around these play areas were chairs, wall outlets, and wifi.
There was also a dark sleeping room for kids, no adults allowed. After a 9 hour overnight flight our kids needed to run and climb more than sleep.
There was another back room for teens. It had bean bag chairs, video games, pool tables, etc. It wasn't crowded when we were there. I would say 5-6 in the teen area, 4 kids playing in the play areas, 3-5 adults watching the kids, and 1 kid sleeping. This was a great find and I wish all airports had something this family friendly for travelers.