Bad Hair Day by Sarah Mlynowski

It's book #5 of the Whatever After Series. 

After a tough day at school, I am NOT in a good mood. The perfect pick-me-up? A trip through our magic mirror! When my brother and I -- plus our new dog -- end up in the story of Rapunzel, we can't resist climbing her hair. Oh no! Her smooth locks suddenly look like they've been attacked by a cheese grater. Maybe a trim will help...Snip, snip. SNIP. Oops. Now Rapunzel's hair is too short and we're trapped!

So we have to:
-Find the tower's secret door
-Avoid getting swallowed by a giant spider
-Locate Pickles (aka the prince)
-Reunite Rapunzel with her parents

If we don't untangle this knotty tale soon, we could be stuck in this mess for good!