You could use a dark wood buffet since the Mackenzie Childs table and chairs already have color, or you can use one of those colors or even another color of approximately the same intensity. Do you have photos of the room and the dining set? Depending on what the dining set looks like, something like the piece below could be fun. The white MC set I saw online was rather whimsical.

I think Mackenzie Childs is a great look ~ a buffet table in a painted distressed finish would work well. I would like to see a strong color so as to hold its own with the MC. I am having a dining room table made with this finish below, which is really a gorgeous version of turquoise !!

I should say this is a custom finish on a custom made table but you can also take a more standard piece of furniture and add an outrageous finish like this from our great craftsmen in Naples, Florida

I feel that the coastal cottage look will be fine. Here in Naples I have seen a lot of that furniture. It is actually easy to mix many thing with it.