A man smacked a toddler so hard that he ended up with "a very obvious hand print" on a buttock.
The assault happened while Christopher McLaughlin had temporary care of the two-year-old. It left him facing a judge at Nottingham Crown Court.
He was ordered to do 75 hours unpaid work after he admitted assault causing actual bodily harm. He must pay £1,000 prosecution costs.
Judge Steven Coupland told him: "Thankfully he suffered no lasting injury but a red mark which faded quickly.
"He seemed fine and there is no suggestion you have done anything like that before."
He said that the offence might lead to McLaughlin being banned from working with children or vulnerable people.
Mark Watson, prosecuting, said that the incident took place in May last year. "A rather large mark" was seen on the child's buttock and it was "a very obvious hand print."
Mr Watson went on: "There is no indication he was in any discomfort."
Soon after, McLaughlin claimed that the child had fallen over. Mr Watson told the judge: "He was getting his excuse in early."
He claimed that the child had fallen into a radiator and gave that explanation to police officers when first interviewed, added Mr Watson.
McLaughlin, 33, of West Avenue, Stapleford had never been in trouble before. He promptly admitted the charge, the court heard.
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Errol Ballentyne, mitigating, said the child was still being potty trained and caused a mess which needed clearing up.
The child was "rather wriggly" and McLaughlin accepted that the blow was "too hard."
Mr Ballentyne told the court: "It was not an area he was necessarily gifted, never having children of his own. He is ashamed of his behaviour and remorseful."
McLaughlin is "a giver rather than a taker" and acts as a volunteer for cancer charities. He supplied several references and clearly "acted out of character," said Mr Ballentyne.
"He has reflected daily on that day and would wish to try and find some way of turning back the clock," added Mr Ballentyne.
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