Hi Antonio,
Thanks for visiting the Disney Parks Moms Panel with your question. We took our daughter to Walt Disney World for her first trip when she was seven months old, and it poured rain on our first day there! Let me give you some thoughts on traveling with an infant in possibly rainy weather.
First, since you want to give priority to Magic Kingdom, I would say go there on your first day. Since you have five days at the Walt Disney World theme parks, you can plan one day for each theme park and then revisit the one that you want to see the most again on your last day, which sounds like it may be Magic Kingdom.
My favorite theme park to visit in the rain is Epcot. This is because at so many of the pavilions you can go inside, and then there is a lot to do once you are inside of the pavilion. For example, if you ride The Seas with Nemo & Friends, then you can spend time exploring the amazing aquariums and aquatic animals inside of this pavilion. You can also see Turtle Talk with Crush, and easily spend well over an hour inside and out of the rain. On rainy days, I suggest that the adults wear rain ponchos. They fold up very flat to be stashed inside of a backpack. I take a gallon-sized plastic zip-top bag to put these in so that they do not get the rest of the items in my backpack wet. When our daughter was an infant, we invested in the rain cover for her stroller, and it was definitely a good idea. That way we could push her around the theme parks without her getting wet. If you need an escape from the rain at any of the Disney theme parks, check out the
Baby Care Centers. These facilities offer a place to change and feed your baby outside of the hustle and bustle of the theme park.
I hope that these thoughts are helpful for you, Antonio. Have a magical vacation!