I would say so. They were being given dialogue in the 4th season when they were only 3 years old. There's a 4th season non-Darrin episode that revolves around Tabitha and she has quite a bit of dialogue (she turns a play date playmate into some sort of creature - I forgot exactly what). They seemed to have advanced the character's age slightly, but not radically so, unlike many other series. I guess the Murphy twins could handle more acting wise than the twins who played to Adam who by the last season could've conceivably been given dialogue but to my memory never said anything on the show.

...in one episode from the third season (I believe), the scene opens with Samantha and Tabitha returning home from a shopping trip. As Sam is talking to Endora, Tabitha starts toddling towards the kitchen. IN CHARACTER, Liz gently guides Erin {Diane?} back towards the camera without missing a line of dialogue, even ad-libbing a bit about Tabitha's sudden departure! For some reason, William Asher thought this bit of impromptu business was charming enough to use without a retake!!

Yes, the Murphy twins could be "precocious"...and professional when they HAD to be.

:tv:

I thought the Murphy twins and Erin in particular did a really good job on Bewitched. The episodes I have seen have only Erin in them and she delivered her lines with zest, I hate seeing kids on tv shows delivering flat lines.

Tabitha starts toddling towards the kitchen....

Yes, the Murphy twins could be "precocious"...and professional when they HAD to be.

:tv:

TVKF, the definition of "precocious" is widely mispresumed. Take a look at the nature of the top post again, then please see the following:
http://m-w.com/dictionary/precocious

Erin Murphy was one of the best child actresses I've seen on TV. Not only did she look like her TV mom (and still does!), but she was cute and charming without being sappy or obnoxious. That's an achievement in and of itself.