A court heard the youngster, who cannot be named because of his age, describe how he was forced to take cold showers if he wet the bed.
He said he could end up shaking during the experience, was called “dirty boy” and was made to drip dry on a towel in a corridor.
A jury also heard him describe the moment he realised Liam was dead, telling interviewers: “I saw it.”
Liam’s mother Rachel Trelfa or Fee, 31, and her civil partner Nyomi Fee, 28, are on trial at the High Court in Livingston, where they deny murdering the two-year-old and falsely blaming his death on another young boy.
The jury has been shown various pre-recorded interviews with the child they are alleged to have wrongly blamed, which were taken in the days and weeks after Liam’s death at a house in Fife on Saturday 22 March 2014.
He is the same child who previously claimed he “strangled” Liam days before his death - but that the toddler was still sitting up and watching television afterwards.
Yesterday, the seventh day of the trial, the jury heard the boy tell a police officer and social worker that he was not allowed to get up to go to the toilet during the night, which sometimes meant he would wet the bed.
He would then “have to get a shower, a cold one” as a result, the court heard, and he could also get a smack on his bottom which left it “red”.
The child told how the showers would leave him “shaking” and were not for the purpose of getting washed. One interviewer asked him: “You were getting a cold shower because you wet the bed. Do you think that that was a punishment?”
“Yes,” replied the boy.
The trial continues.