If a child has FOP, their parents would know it. The bones of the feet develop last, and if you try to do surgery to correct their bunions, it is likely the deformity will come back with vengeance.
Never seen one as this is exceedingly rare, but if there are any questions about whether a patient has it or not, send them to a pediatric rheumatologist for a thorough evaluation. It is a very rare connective tissue disorder, so it may be difficult to find someone who can help. Also, as you mentioned it is incurable.
