APPLETON, Wis. (AP) _ The 14-year-old son of a baby sitter charged with murdering three children in her care testified Tuesday that he had covered a baby's mouth and tied a baby up, but did not cause the deaths.

Christopher Pankow, who has been granted immunity from prosecution, took the stand shortly after his mother, Sandra Pankow, 36, denied she had caused the deaths of the children over a five-year period ending in 1985.

The teen-ager was called to the stand by defense attorney William Wilde and asked whether he was jealous of the attention his mother paid to his learning- disabled younger brother, Aaron, and to 14-month-old Kristin Hamilton and 9- month-old Shawn Bloomer. The Hamilton girl died Dec. 19, 1980, and the Bloomer boy Aug. 10, 1982.

Christopher said he had been jealous. Under questioning, he also said he tied up babies at his mother's house and covered up a baby's mouth.

But Christopher said no when he was asked whether he thought his mother had done anything to cause the death of the Hamilton and Bloomer children or 6- month-old Tyler J. Kloes, who died at the Pankow home Oct. 25, 1985.

The defense attorney said the testimony was not intended to suggest Christopher killed the infants.

The defense has contended that the deaths were a result of sudden infant death syndrome.

''I think we have two phenomena here. We have some abusive conduct and we have three SIDS deaths,'' said Wilde.

Dr. Fred Zugibe, chief medical examiner of Rockland County, N.Y., testified Monday that that the discovery of blood in the lungs of the three infants indicates the deaths were homicides. Zugibe agreed with previous prosecution witnesses who said the children died of asphyxia, but added that the asphyxia was caused by external force.

Christopher's mother testified earlier that, periodically, she left children unattended for as long as 30 minutes while she went to talk to a neighbor.

''Chris would check on the children then,'' she said.

Mrs. Pankow testified that she had put the Kloes child on a freshly- vacuumed carpet the day he died. She said that, when she went to pick him up again, the tips of his fingers had changed color.

''I picked him up and cleared his airway and started mouth-to-mouth'' resuscitation, she said.

Mrs. Pankow denied doing anything to cause the deaths of the three children. She denied she ever put anything over the mouths of the babies.