My kids were 2 & 4 on their first WDW trip and 2 and 4/5 (5th bday celebrated on site) on their 2nd trip. We hit the parks 6 days on their first trip...3 days in MK and 1 day in each of the other parks. On their second trip, we only had 3 days in the parks...and we did MK, Epcot, and DHS...opting to skip DAK. We would have preferred to hop between DAK and DHS, but it was just too much $ to justify adding 1 day as a hopper.
All this being said, I wouldn't just do all MK. If you're planning on 5 park days...I'd probably do 2 days at MK and 1 day at each of the other parks (assuming I read the # of days correctly).