I say just get to the MK as early as you can, then hoof it on back to the fairies first thing. Maybe you can get in and done before the party even starts. Just make the most of it, bring in a snack to eat while in line. My daughter and her friend played little games with the other girls in line, so it passed quickly.
If you haven't done it, then I really recommend it.
As for the princesses, the princes won't be with them until after the party starts at 7. If you're just getting out from seeing the fairies and it's close to 7, you might want to ask if the princes are there yet. If the line's not too long, it might be worth doing.....