My 6 and 10 year old daughters are not very adventurous when it comes to scary rides -- would Knott's Berry Farm be a good time for them?

I think it'd be fine. Camp Snoopy has lots of little kid rides. And they used to have some good shows.
Disneyland has even more little kid rides.

frankie's 100% right. Knott's is fine for little kids. Disneyland, with all the character-based activities and "little princess" stuff, would fit them to a T as well.

We really loved Legoland, our daughter as well as our sons. I know it's a little farther away but if you're renting a car, it's no big deal. And it's not just for Lego fans.

Highlights for us:

The Volvo Driving School was very popular with my family -- the cars are not on a rail or track. The kids really drive them, with stop signs, lights etc. The Royal Joust was reallllly popular for my younger two; they did it many times.

At the Fun Town Fire Academy, your family races an old fashioned pump and ladder together. The Aquazone Wave Racers get the spectators involved, really fun. The musical fountain was very creative in the younger kids' area.

My oldest was then 8 and did the robot programming with his dad. (You sign up when you first arrive, very easy).

http://www.legoland.com/

I think Knott's is perfect for younger kids. Camp Snoopy, catering to the younger ages, is a safe haven for them. Since it is not as congested and not as huge in land area as Disneyland, it is much more relaxing and enjoyable, and less stressful. Don't get me wrong, Disneyland is fun too, just a different type of experience.

You might want to avoid the log ride, although it's my own favorite.

My children have always enjoyed Knott's and they have some really cute shows. I think their rides are less scary than Disneyland's. They have mellow rides without all the dark, loud, "evil" themes. Have a great time!