The 29-year-old freelance photographer based in Nuneaton, Warwickshire, caught the judges' attention with his image (above) of Gaby Allen, aged five, taken during the Queen's golden jubilee celebrations in May 2002.
Mr Sewell was covering a Kings' and Queens' day at Hugglescote primary, Coalville, Leicestershire. He said: "Gaby was the best-dressed there.
Children naturally smile in photos and the hardest part was stopping that.
I told her to pretend she was the Queen and eventually managed to get her with a straight face."
Mr Sewell started taking photos when he was 10. He studied press photography in Sheffield before working at a Birmingham news agency. He turned freelance five years ago.
The judges were Eamonn McCabe, former picture editor of the Guardian, Chris Ison of the Press Association and Jonathan Eastland, the veteran photographer.
Abigail Hadeed, a Trinidadian, won first prize for her striking photograph of two young girls in Guyana.
The Commonwealth Press Union includes more than 1,500 newspapers and agencies. It aims to monitor and campaign for freedom of the press.
The winning photos are being exhibited at 18 Northumberland Avenue, London, until September 23