My suggestion would be to get to MK before opening and wait over to the left side entrance and head for the barbershop as soon as the ropes drop. Even if it's not open yet it will be within 5 or 10 minutes and you won't have to wait!

We got DS2 first haircut there in January and that's what we did. We were the first people into the barbershop and there was a CM getting his haircut already in the other chair. Both my boys got their cut by the time all was said and done we were in there maybe 20 minutes tops and when we left there was a HUGE line outside. I talking a 2 hour long line! YIKES!

If going first thing isn't possible I also heard that it slows down later in the day.

No matter what it's definitely worth doing! enjoy!