Frogkisser! book by Garth Nix

The last thing she needs is a prince. The first thing she needs is some magic.

Poor Princess Anya. Forced to live with her evil stepmother’s new husband, her evil stepstepfather. Plagued with an unfortunate ability to break curses with a magic-assisted kiss. And forced to go on the run when her stepstepfather decides to make the kingdom entirely his own.

Aided by a loyal talking dog, a boy thief trapped in the body of a newt, and some extraordinarily mischievous wizards, Anya sets off on a Quest that, if she plays it right, will ultimately free her land—and teach her a thing or two about the use of power, the effectiveness of a well-placed pucker, and the finding of friends in places both high and low.