[MysticCat]

Or Wikipedia:

The producers admitted that they did not think audiences would notice Judy's disappearance, which is why her absence was never explained. Later episodes even have the Winslows acknowledging they only have two children, not three. The last time Judy was ever seen on the show was when she went up to her room. But she never came back down. According to Jamiee Foxworth who played Judy, she's "still up in her room putting clothes away".