Can you do a kind of test run by taking your kids to visit the relatives in CA for an extended time over the summer? If you do, can you stay in an apartment or "residence inn" type of hotel so you're not staying with relatives? I mention that because you could learn a lot about the realities of traffic, cost of gas day by day, cost of groceries, etc. if you are in the area and close to your family but not staying with them, not using their cars, and shopping and making some meals on your own temporary turf. Visiting with people is nice but does not give the visitor a realistic idea of what it's like to live in a place (and if you lived there a while back, it won't be the same now, so you still could benefit from going out there but not staying with the family).
Also, I'd really check out the family situation out there. It's fantastic that the relatively young grandparents would be able to see more of the kids! But you do need to learn things like how involved they are in other stuff (for instance, if they are active seniors, they may not have as much time to babysit as you -- or they -- think they will). You might find that other relatives you know are not going to be as available to be in your lives day to day as you would like. Remember, when you visit, you're the center of attention and folks naturally go out of their way to be with you, but when you live in a place it's different. That's fine too! But it's something to consider. Also, your kids' cousins may or may not end up as their friends. Just being blood relatives does not necessarily mean they'll all grow up as buddies or share interests. That's just natural, but if you picture your kids growing up with heaps of close relationships with relatives, just be aware that it may or may not happen; you need to be happy in the area with or without that as a factor in a move.
The schooling issue is crucial and only some time spent out there, and a lot of online research, would answer some of those questions.
I think you have some great circumstances, especially the fact that your husband's company would make such a move pretty easy on the work front. I just hope you can step back a little from BOTH the emotion of "I want a big family around us" AND the emotion of "I'm scared to make this move" and get some objective perspective.
One last thing. You mention you have one child and may or may not have more and you want your child to have family in her life if you don't have another child. Please don't fall into the trap of "I have an only child and must provide family for her beyond her father and me or she won't be complete or happy." Neither siblings nor cousins are essential to a child's happiness. Be sure you're making a life for her here and now, so that even if you don't move, your family of three IS complete and happy.