Hi, V., I do not have personal experience with my children in either program, but I do have experience as a teacher (with older students). I believe that most K12 schools will not enroll students in the middle of a school year, so you would want to be prepared to check with California to see if that will be a possibility or plan on enrolling them at the beginning of the school year to begin with. Typically you need to live in that state to be able to enroll in the program.
K12 for the lower grades is going to require a lot of parental involvement. In other words, no one can expect a 1st grader to sit down at a computer by him/herself to learn, so the parents are a huge part of that guiding process. My experience with their curriculum is that it is top notch...I say this because most states are VERY strict when it comes to allowing an online school to become accredited (which K12 schools ARE accredited). Basically, most states don't want just anyone to be able to open up an online school, so they make it really tough to get accredited. All of your child's teachers will be licensed to teach in that state, and speaking from my own state, we have some of the top educators (because a lot of people want to have the opportunity to teach online...meaning that only the best are kept! ...and there is no teacher tenure with K12 schools).
As I mentioned, my experience is more with the older students, but we do have 'live' classes where all students log into a special program and we can all talk to each other, look at lessons on the white board, etc., all at the same time to get some good interaction. There are also lessons in a separate location where students complete items to show their understanding (this is usually something that is done at the student's own pace, with an end date for completion).
I'm sure that homeschooling (when done properly) is a lot of work...Regardless of whether you use K12 or some other program, you'll want to be prepared to spend lots of time working with your kiddos (and I'm sure you are!). :)