Weren't they considered old to have children at their age, back in the mid-1960s? And why was Jonathan Tate (https://bewitched.fandom.com/wiki/Jonathan_Tate) only seen / mentioned in three episodes? Why didn't the Tates ever bring him to the Stephens' house when the visited, so he could play with Tabitha?

It seemed (to me) to be suggested that it just never happened and the Tates thought they could not have kids. When Samantha asked her about it she said "yes finally" like they had tried to start a family unsuccessfully for years.

That's a very well-reasoned answer tcr1701.

From a series practicality standpoint as well, when it was needed for the Tates to have a child for a storyline, they had one. Later writers would find that of no use or concern, presuming they even knew the infant/toddler had ever been introduced earlier in the series...!

Larry "Cotton Top'' seemed to have a roving eye "you sonofagun".

I always found it hilarious when Serena would come-on to or flirt with Larry. I can't imagine in real life having Liz Montgomery acting like that toward me, I would probably pass out.

I think EM really got into the Serena character.

As Barbara Eden did the same with Jeannie's sister.