This is a very sweet and interesting story. Quite original too, I thought. However, there were a couple things that bugged me. First of all, it ended on a cliffhanger. A 112-page children's book should not end on a cliffhanger.
The other issue is that it's very unrealistic. The castle has one cook, a housekeeper, a little girl who is the only maid (who does pretty much everything), and a little boy who does all the gardening AND takes care of the stables all by himself. The only other people who live at the castle are the queen, one knight, and some guards. At one point the girl "climbed the fence into the field next door." Because castle grounds are apparently surrounded by little picket fences.
I know it's a children's book, so I am being more lenient in my rating, but I think we owe it to our children to treat them like the intelligent young people they are. It would have been very easy to include a sentence saying "Castles are very big, with many rooms to clean and many people to feed, so it takes a lot of servants to take care of such a huge place."
We don't have to meet any of the other servants, but don't make it sound like one little girl cleans the entire castle, does the laundry, has to carry a cartload of furniture outside, has to pick the apples in the orchard all by herself, and then even helps the cook! Even if you don't know that castles are huge, even if we were only talking about a regular house, that's too much work for one person, especially a child, and kids are gonna notice that.