A trial heard how the child's mother went out for the evening, leaving 25-year-old Scott Bambrick in charge of her daughter and two-year-old son.
She returned to her Stirling home in the early hours of the morning and was woken next day by the little boy.
The mother was alarmed to discover her son's nappy was fastened with masking tape. She found that both he and his sister had badly marked bottoms.
Doctors examined both youngsters and concluded that the injuries were not accidental. Some of the bruises appeared to be fingermarks, the High Court in Edinburgh was told. The little girl also had other more suspicious bruising.
Bambrick, of Stonehouse, Hamilton, claimed he did not know how the children were injured but admitted to police that he had given the little girl "a wee skelp". He also said he had smacked the little boy's hand.
In court, Bambrick lodged papers blaming their mother for anything which had happened. But a jury rejected his denials and also his suggestion that the little girl could have fallen against an upturned table leg.
Bambrick was found guilty, unanimously, of striking both children with his hand on the night of May 19 last year.
Source: STV News
