Roppongi 六本木, Minato-ku 港区
Roppongi is not a very easy place to go with kids. It’s crowded and busy like Times Square and it’s generally not a children’s area. But there are a lot of families who live in the area so you can find parks and playgrounds. If you must go with infants and/or toddlers I find Roppongi Hills to be a rather child-friendly area.
There are a couple of children’s shops, lots of open space to run around, an outdoor event area where they sometimes hold concerts or special promotional campaigns, a 
lovely little Japanese Garden, the Mori Art Museum and the TV Asahi Building with large Doraemon figures. Roppongi Hills also has the best family room I’ve seen. It may not sound so exciting, but when you travel with two very young kids this kind of free facility is such a blessing and I wish there were much more like it all over the world. We often eat our bento lunches there if it’s too cold outside since there are kid-size tables and chairs, some toys, and the room for breastfeeding is also right there. Plus, the room is clean and cute. Just too bad the big plasma tv on the wall never plays anything good, just wasted on Roppongi Hills advertisements. If you continue walking towards the residence buildings you can also find a playground with rows of crazy slides.
To get to Roppongi Hills take Oedo line to Roppongi Station. There is an elevator from the station to street level. From the station locate the tall Mori Building (building with a big M at the top) and it’s about a 10 minute walk.