I completely agree with Mickeygolf! If you want to stay in the "Disney bubble" and you have everything you're gonna need and aren't traveling off site, I would definitely not rent a car.
We usually drive down and then we park our car while at POR and don't drive again til we leave the resort. For us, it's part of the experience. My kids are a bit older but we've been doing this for years. If you're kids are fussy or particularly tired and you are standing, very often someone will figure out a way for the young child to sit.
I also think it depends on your mindset. If you are someone who worries about that extra 10 or 15 minutes that may happen at times, then a car may be best for you. When you relax and just go with it, it is so much more enjoyable.
With young kids, if you are not on the dining plan and want to be able to run through McDonalds or the store, you may want a car but it isn't necessary at all if you plan.