Really tiny cx frames are (or at least used to be) difficult to find. I know that Ridley made the X-fire in a nominal size 41 (ett around 50). We had a pair and when I sold them it was almost all parents of Jr riders who were interested.
Good luck and hope your daughter crushes it out there.
The geometry on a MTB frame blows for cross. The bb is usually way too low which means you'll be whacking your pedals on the ground on every turn. Especially with cross tires on.
Maybe ok for a gravel bike, but not worth it for a bike you plan on racing cross with.