Just like an adult, it's going to depend on the face size /shape...
Try 'em and find one that you like... As Lee recently realized, if you
have a mustache, it's not a matter of *whether* your mask leaks, but
if you are willing to put up with the *amount* that it leaks... I've
given up trying to find a mask that won't leak... On the other hand, a
leaking mask just means that you don't have to worry about it fogging
up as much since the water washes the fog away... <grin>

You already figured it out. Fit is the only criterion.

There is no relation at all between fit and price, so when trying masks
don't bother looking at the price tags. Find one that fits and pay
whatever the bandit asks. Of course, if you are lucky enough to find two
that fit, buy the cheaper of the two.

esg

1.  Go to a real dive shop.
2.  Put a mask up against your face and inhale through your nose.  Look down
and if the mask sticks it fits.
3.  Now smile and inhale.  If you can feel air being pulled in it may leak a
bit, or don't smile when you dive.  Smiling will make most masks leak.
4.  The nose pocket should not press against your nose, and you should be
able to easily grab your nose to equalize.

Now, find the most comfortable mask that "fits" and buy it.  True, you will
pay more at a dive shop, but you'll have the opportunity to try a lot of
different masks.

Also, as someone else said, a little leakage isn't too bad as long as you
know how to clear it. If you are just going for your certification many
instructors, especially if they work at or own dive shops, will bring a
bunch of masks to the pool if you ask.