The Wingless Fairy Series, Book 1: Rebecca and the Changeling by Margaret Pearce

Princess Dewdrop is bored out of her mind.

Life in fairyland is an endless parade of moonlit revels, lessons she refuses to attend, and nothing interesting to do. When the house goblin Golly reveals that a human baby has been stolen and replaced with a changeling, Dewdrop sees her chance. She hatches a plan to put things right, and drags her long-suffering guard Lord Be Thankful along with her.

But fairyland has rules, and breaking them has consequences.

Trapped in the mortal world after eating mortal food, Dewdrop must find a way to reunite both families before the full moon rises. Hobgoblins are circling. The Fairy Queen's fury is building. And the only person who can complete the swap is a young farm girl who doesn't even believe in fairies.