As far as I know, UK child passports are valid for 5 years, with no exceptions.
However, I got a passport in (I think) 2004, shortly before my tenth birthday, which didn't expire until 2014. By that point, as you might expect, I looked very little like my passport photo, which depicted a nine-year-old, but my passport was still valid. I even managed to get into the United States at JFK airport with it.
Obviously, the passport has expired by now, and I currently have a normal 10-year adult passport. My question, asked purely out of curiosity, is, how did this happen? Are there any other cases of children being given 10-year passports? What are/were the criteria?