Hayley Turner, from Oxfordhire, believes "lowlife scum thought it would be 'fun' to deliberately put drawing pins in the bottom of the changing mat" at her local village hall.
Turner's son Samuel screamed as she placed him on the mat but she dismissed it as him being tired, cold and grumpy, "because he isn't old enough to tell me it hurt."
"He now looks like a pin cushion with five holes in total," Turner wrote on Facebook, as well as sharing pictures of her son's cuts.
"Angry doesn't even cover it. It's quite sad to think this has to be done but my word of warning is to check your changing mats."
Police are now investigating the incident.
"I don't normally post things like this but I'm guessing if it happened to me it could happen to someone else, so I thought I'd spread a little word of warning to prevent it happening in the future," Turner's Facebook message began.
"Today I changed Samuels' nappy on a public changing mat, somewhere I trust to be a safe place.
"When I led [sic] him down he screamed. I dismissed it as him being tired, cold and grumpy so I changed him as quickly as possible so I could give him a cuddle and calm him down.
"It turns out some lowlife scum thought it would be 'fun' to deliberately put drawing pins in the bottom of the changing mat. Our poor baby boy was led [sic] on pins."
Turner's husband Rob later took to Facebook to reassure friends and family that Samuel is now "his normal dribbling self", but he was "fuming".
"Poor innocent baby," he wrote. "What was going through the mind of the person that did this? My poor boy. Glad he's ok now and hopefully easily forgotten and not left with mental scars."
Mr Turner added that he doesn't hold the village hall responsible.
"Not their fault really as so many people use the hall," he wrote.
Thames Valley Police told ITV two babies had been injured in the village hall in Rack End, Standlake, Oxfordshire.
"It was reported that drawing pins left in a changing mat had injured the babies," Thames Valley Police said in a statement.
"Police are investigating this assault and in the meantime safety advice has been given to those involved."