Thomas Poole, 27, hit the three-year-old boy last March 6, at a flat in Whiteinch, Glasgow.
He lost his temper and smacked the child twice on the back, causing him to fall and hit his chin, which resulted in the bleeding.
The youngster – who cannot be named for legal reasons – also suffered bruises on his bottom, lower back, stomach, legs and chin and was kept in Yorkhill Hospital for 10 days.
Poole, of Mosspark Drive, Cardonald, pled guilty at Glasgow Sheriff Court to repeatedly hitting the child on the body to his severe injury.
The court heard Poole watched the boy on March 6. Procurator-fiscal depute John Bedford told the court: "The accused sent the child's mother a text advising her 'he fell a cracker today and has the worst bruise iv ever seen on his bum'."
Mr Bedford said: "On arriving home, his mother found him to be lifeless and quite sleepy. On lifting his pyjama top she noticed that he did have significant bruising."
The next day she took the boy to Yorkhill Hospital. A doctor noticed bruising on his bottom, back, thighs and front of both legs and abdomen that were not thought to be accidental.
Defence lawyer Callum Weir said Poole lost his temper and hit the child twice on the back.
Sheriff Martin Jones QC deferred sentencing for background reports and remanded Poole in custody.