When DD was younger we did it in the middle of the trip on our second MK day. We always did MK on arrival day, then MK RD the next morning, mixed it up with other parks in between (we do an AM park and a different PM park) and by day 4 or 5 went back to MK for RD again. This way our first MK morning was dedicated to soaking in the atmosphere and doing rides. We had DS who is 6 years older and we wanted him to enjoy the parks too. He probably would've been super antsy to wait at BBB very early in the trip when the park is out there waiting for him! We also always ate at CRT lunch the day we did BBB so it ate up a good amount of time.