I think that if your kids have a passionate love for, say, geometry, then you can feel free to start with any of the geometry books.
Correct me if I'm wrong and just haven't gotten to one for which this is not true, but in my experience, the MM books really stand alone quite well. It's not a curriculum, where A must come before B which must come before C. Having some background is going to help, obviously, but provided you're available to answer questions, I don't think order matters a whole lot.
Though, again, I could be wrong because we haven't read them all yet...
