When my kids were that age they were very uncomfortable in a poncho and preferred a regular kids raincoat. I just bought the ones from wallmart or target and brought them with me, somtimes disney themed sometimes not. Use a large ziplock and you can fold the raincoat almost as small as a poncho and use the ziplock to carry it when necessary. It also makes it easy to see your kids in the sea of mickey ponchos. My dd had a princess atta raincoat that was bright purple and I could spot her a mile away!!
If you are using a stroller we used the dollar store clear poncho's to cover the stroller, especially in the afternoon if we were going to spend some time in an attraction. You can get a quick downpour without warning and its nice to have a dry stroller.
If you are going in spring, or summer boots may not be necessary. It only showers for a short time and my kids would be to hot in rainboots. We wore good fitting teva or merril sandles in summer trips to keep cool, comfy and not worry about the showers.
HAve Fun
TJ