We did MK in grades 1 and 2. We didn't do much preparation, but I guess it depends on whether you have expectations about doing well. We did free sample problems over a handful of sessions, and worked with Challenging Word Problems as part of our math. He finished MM 4 for 2nd grade, so not as advanced as yours, and sometimes I'm not sure he has retained anything.
At the center we take it in, it's in a classroom at desks, filling out a "scantron" type form. I think you have 75 minutes, but my son has been out the door after 15-20. I keep hoping he'll slow down and be more careful. Our testing center is an hour away and I take him out for sushi and ice cream after to make it a special thing.
We tried CML this past year. It is really low key as you can take it at home. But with it being at home, my son was like, "is this a real test?" and I'm not sure he put in his best effort.
ETA: thanks for mentioning Caribou. That wasn't on my radar.
Edited by debi21, 31 July 2017 - 06:19 PM.