Convicting the man and deferring the sentence for a year was a ''difficult decision'', he said.
''We require the highest standards from foster carers in this country for these vulnerable young people," the judge said.
The defendant asked the child, who was in a bedroom, if he had soiled himself. The child denied it but the defendant found faeces and soiled clothing on the floor and ''lost his temper'', Judge Flatley said.
The man rubbed the clothing on the boy's face, saying ''you know better than that''.
It was an unpleasant form of assault and a breach of trust on a ''vulnerable and defenceless'' victim, the judge said.
Counsel Russell Checketts asked for the defendant to be discharged without conviction as the consequences of a conviction would outweigh the gravity of the offending.
Judge Flatley said there was no evidence of that.