can we take the kiddos just to Disney (not epcot, MGM or anything else) and do just 3 days or is that not enough time
You definitely can't do all 4 Disney World parks in 1 day, but 3 days is a good amount for young kids to do just Magic Kingdom definitely.
I took a 5 year old and stood in line for 2 hours at the Bippity Boppity Boutique just to meet 4 Disney princesses. So in short, I you plan on really doing most/all the rides and experiences as opposed to just kind of breezing through picking and choosing, you need more than one day but not more than 2-3.
There is some cool stuff at some of the other 3 parks and in Orlando in general. Like I don't know what baby Plank is into but at Islands of Adventure (Universal Studio's other park) there's a whole Marvel comics section, the Wizarding World of Harry Potter is in there, the Simpson's neighborhood is in there, all with rides. Not knowing him, I think most boys his age would love that park.