I noticed a couple of trends in certain 90's family sitcoms and wanted to list them. Actually, I noticed these things long ago but never had an outlet to it. By the way, many people consider these examples of 'Jumping the Shark' for the series.
Mommy looks different
Family Matters' Jo Marie Payton who was the character Harriette Winslow that got spinned off from Perfect Strangers. Once the show moved from ABC to CBS for its last season, she left because she didn't like that show focused on other characters other than the original premise (a blue collared Cosby Show) and the part went to Judyann Elder (who I can't find a picture of). Jo Marie also wanted to direct and produce but was never allowed to. This is all alleged of course, I don't know the truth or claim to. Of Course, many people may not know she was replaced since the last season on CBS was little remembered.
Secondly The Fresh Prince of Bel-Air where the mother Vivian Smith-Banks was originally played by Janet Hubert-Whitten and during her pregnancy, she had some disagreements with the producers, namely by rumors Will Smith, and she left the show. Daphne Maxwell Reid took her place. James Avery (pictured above and below) also made a cameo in Family Matters when the resemblence to him and Reginald VelJohnson's Carl was mentioned.
I gave birth to a five year old
On the show Fresh Prince, Will got a baby cousin in season 3 named Nicky and he went from infant to pre-schooler from seaosn 4-5, played by Ross Bagley who also played Will Smith's step-son on Independence Day. The rapid growth was slightly addressed by Will 'breaking the fourth wall.'
On Step by Step, the show about two divorcees joining their pair of three children together in a Brady Bunch fashion. The tension broke between the step-brothers when a child of both parents was born. Emily Mae Young played Lilly, who magically grew up in season 6-7 after being born at the end of season 4. Emily was best known at the time for Sears and Welchs commercials.
On Growing Pains, Chrissy also jumped three to five years in season 6 played by Ashley Johnson after being born at the beginning of season 4.
On Married with Children, a cousin Seven (Shane Sweet) was adopted and because of the character's unpopularity and the writers not knowing what to do with him, he was dropped and was a slight running gag. The show's creator even said 'he went upstairs and never came back down' on E!'s THS.
The child magically growing with no reason is often done in soap operas (but one wonders why if soap operas last for decades) for writers to have more to write about and gain more interest. Some spawn on television who actually get to grow naturally are (to name a few) the twin spawn on 7th Heaven, the twins of Jesse and Becky on Full House, and Richie from Family Matters. One growth that went from infant to older child that was a neccessity was on Angel, where baby Conner became a teenager by spending excelerated time in a 'hell dimension', this is explained better in Science Fiction shows. Another who grew fast on that show was Conner's 'daughter' Jasmine who was just born as a grown woman. In 4400, baby Isabel goes from infant to young woman in season three after being born at the end of season one. A valid explaination for her growth was that she sucked the youth out of her mother, her mother becoming an elderly woman who eventually died.
On the show Fresh Prince, Will got a baby cousin in season 3 named Nicky and he went from infant to pre-schooler from seaosn 4-5, played by Ross Bagley who also played Will Smith's step-son on Independence Day. The rapid growth was slightly addressed by Will 'breaking the fourth wall.'
On Step by Step, the show about two divorcees joining their pair of three children together in a Brady Bunch fashion. The tension broke between the step-brothers when a child of both parents was born. Emily Mae Young played Lilly, who magically grew up in season 6-7 after being born at the end of season 4. Emily was best known at the time for Sears and Welchs commercials.
On Growing Pains, Chrissy also jumped three to five years in season 6 played by Ashley Johnson after being born at the beginning of season 4.Disappearing Children
The above series mentioned also suffered from removing a character while adding new ones or loosing interesting. On Family Matters, youngest daughter Judy (Jamiee Foxworth) 'went upstairs' and never came down again. The character simply was never popular and writers couldn't come up with anything for the character. She recently appeared in an E! special.
On Step by Step, Patrick Duffy's TV son Brendan Lambert (Josh Byrne) disappeared once focus went to baby Lilly and other comic relief characters.
On Step by Step, Patrick Duffy's TV son Brendan Lambert (Josh Byrne) disappeared once focus went to baby Lilly and other comic relief characters. 
On Married with Children, a cousin Seven (Shane Sweet) was adopted and because of the character's unpopularity and the writers not knowing what to do with him, he was dropped and was a slight running gag. The show's creator even said 'he went upstairs and never came back down' on E!'s THS.The child magically growing with no reason is often done in soap operas (but one wonders why if soap operas last for decades) for writers to have more to write about and gain more interest. Some spawn on television who actually get to grow naturally are (to name a few) the twin spawn on 7th Heaven, the twins of Jesse and Becky on Full House, and Richie from Family Matters. One growth that went from infant to older child that was a neccessity was on Angel, where baby Conner became a teenager by spending excelerated time in a 'hell dimension', this is explained better in Science Fiction shows. Another who grew fast on that show was Conner's 'daughter' Jasmine who was just born as a grown woman. In 4400, baby Isabel goes from infant to young woman in season three after being born at the end of season one. A valid explaination for her growth was that she sucked the youth out of her mother, her mother becoming an elderly woman who eventually died.