Middle school level fantasy where girl can turn into any animal but is most connected to wolf, finds a boy who can as well but gets stuck halfway as a panther

Set in medieval years with 'evil' king trying to take/unalive all the people who have this ability. They can change into any animal but the animals instincts are strong and they have to mentally fight what the animal wants to do, except for one animal theyre most attuned to. Girl(early-teen) is most attuned to wolf and loves baths, boy(7-10?) is most attuned to black panther and has a hideout that has a cold spring. She helps him during his transformation to get him unstuck and thru more shenanigans end up in the kingdom to save someone. She has a wolf companion and both the wolf and panther-boy save her once she gets kidnapped.