We love the Peninsula and they have a particular level of Junior Suite that has a separate living room for your kid that is actually cheaper than the suite that is all one room. I think it is called a Junior Executive Suite. We always get this room. In fact, on our last trip we declined an upgrade to the "better" suite.
Obviously, the location at 55th and 5th is great. They have a great rooftop pool so don't let your 6 year old forget his swimsuit.