Now that the weather is cooling down, between typhoons it is a fantastic time to enjoy the parks with your kids. Here are 2 English sites where you can find some of the better known and larger parks around Tokyo.
Tokyo Metropolitan Park Association
Tokyo Metropolitan Parks and Gardens List
My favourites are:
Rinshinomori in Meguro-Ku
Kasai Rinkai in Edogawa-Ku
Mukojima-Hyakkaen in Sumida-Ku