[dustycat]

I have no medical qualifications, so take this for what it's worth, but I was told by a psychiatrist specialising in child development disorders that excessive hand-washing is one symptom of Asperger's syndrome.

[tapland]

It is one of the factors that they check for as part of an ASD (Autism Spectrum Disorder) diagnosis, yes. But if it is severe they will strap on the OCD diagnosis as well.

[rubidium]

Not the most likely thing though. kids like playing with water and it gives them something they know how to do and control.