“Well Wished” summary from Amazon.com:

One wish for a lifetime…
In the village of Bishop Mayne there is a magical Wishing Well where a person may make one wish in a lifetime. But the Well can create problems for those who use its power, for wishes often go wrong. It was just such a wish that took all the children in the town away. Only eleven-year-old Nuria, who lives with her grandfather up on the mountain, remains.
Then one child returns — Catty Winter. Catty’s legs are mysteriously crippled, and Catty desperately wants Nuria to make a wish so she can walk again. Nuria swears she will make the wish for her friend. But the Well has a mind of its own. What if Nuria’s wish qoes wrong?