I put in a black/brown (ie, dark) granite in the bath my girls use. I wouldn't do it again--but that is me. Here is why--
The dark shows everything. It isn't varied enough to help hide anything. (Toothpaste spit that doesn't make it into the sink and isn't washed off etc).
Also, it "eats" a lot of light. Mine is the usual high gloss granite finish. I'd think that if you went with leathered uba tuba, it could eat even more light.
That isn't necessarily bad, but it is something to remember. You know, pair it with white cabinets, instead of coffee-stained, etc.
It is a durable granite though--sealing isn't really needed in the dark granites, and it won't "stain". So, factor that in too.
Threeapples, I think you know my bathroom, and I'm not a kid nor so I have any, but I used antique brown in a leathered finish, and I love it. I can't imagine it presenting any problems that would arise with whatever else you picked, and at least you can avoid marble etching which would be a problem (even more so for kids).
I have dark leathered on my teen son's vanity. It shows everything, toothpaste, hairs, water spots. It is pretty....but I think I'd opt for lighter if I had to do it again.