James Brown-Ieve was left with blisters 'the size of tennis balls' by Joanna Mingle, who also hit him so much his face was puffy and he had a black eye, Maidstone Crown Court was told.
Dr Thomas Little, a paediat rician, called the marks 'consistent with being punched and roughly handled'.
Mingle, who James called Nana, was found guilty of cruelty between July and December 1992. She received an eight- month jail sentence suspended for two years.
A doctor had told the jury James's injuries could not have been caused by accident.
Passing sentence, Judge Felix Waley told Mingle, of Hoo, near Rochester, Kent, that James had suffered 'the most appalling scalds'.
'Those in themselves could have been an accident but the other evidence presented made it clear to the jury that it was not. That young boy also had a black eye and other bruises.
'Normally, I would see no other alternative to a custodial sentence. However, I bear in mind the effect this would have on your husband and three young children.'
He added that he hoped no one else would let Mingle look after their children.
Mingle, who denied cruelty, told the court: 'I ran the cold water first and then let the hot tap trickle while I went into the bedroom to clear up the mess he had made. When I came back in, steam was rising from the bath. I don't know how he got the bruises on his face. I guess he must have hit his face on the side of the bath.'
After the hearing, James's mother, French-born Sabine Ieve, said: 'I trusted her with my baby and she betrayed that trust. Every mother feels guilty about having to leave their child, but when something like this happens you just blame yourself even more.'
She said after the injuries were inflicted James became 'just like a tiny baby again' and would not talk properly.
'Eventually one day he said to me 'Mummy, Nana beat me and burned my bottom in the water'. We both cried and cried and since that moment he has never spoken of it again.'
The National Society for the Prevention of Cruelty to Children was surprised at the leniency of the sentence.
Its spokesman, Ron Lock, said: 'If abuse was premeditated, the court should have acted more harshly than it did.
'This woman is a menace and a danger to young children . . . The abuse sounds horrific. When a mother sends her youngster to a registered childminder she is entitled to expect the child to be protected - not attacked and put in a bath of boiling water.'
Kent County Council said Mingle was no longer a registered childminder.Reuse content