In response to your questions:
1)
Disneyland has more rides for kids in the 8 and under age range. Your 3 and 4 year olds will most likely be too short for many of the rides at California Adventure. The rides at Disneyland do not have nearly as many height restrictions as the rides at California Adventure (although there are some, so check their website to know ahead of time which rides your kids will be permitted to ride).
2) Do you have your heart set on doing every tiny little thing? With kids those ages, there will be some things your family likely wants to skip (Indy, perhaps? Space Mountain? Big Thunder Mountain Railroad? Possibly even rides like Pirates of the Caribbean and Haunted Mansion?) If you're not a die-hard Disney fanatic, and if your kids' opinions of what is a fun ride and what ISN'T a fun ride will likely keep you anchored to Fantasyland most (or all) of the day, then 1 day would be enough, IMHO.
Also, with a family with several children, adding more days to your visit gets pricey, quick! It's not just the tickets...it's the extra days at the hotel, food costs, parking costs, etc. Is your visit to Disneyland one part of a larger vacation, or is it the main focus of your entire trip? If it is the main focus, you may want to consider adding a day or two, but again, think seriously about how much your kids are going to be willing to do there (I have many extended family members who, as children, were so afraid of most things at Disneyland that they would not ride them, and some of them even have children now who are afraid of the rides, too). If you feel pretty confident that they will love almost everything and be willing to do a lot while there, then adding days makes sense. But if they are timid and/or do not like to try new things (very common in the 3 to 4 age group...), then sticking with 1 day probably makes more sense. I've visited Disneyland with people whose kids (ages 8 and 10 at the time) were very timid about rides and were to frightened to ride almost anything in the park. For that family, 1 day was MORE than enough time. Staying any longer would have been a huge waste of time and money for them.
3) I've never stayed on-site, so I can't compare. It's far less expensive, and not much more inconvenient, to stay at a nearby hotel, like Desert Inn and Suites (you can easily walk to the park from there). But if budget isn't a huge concern, I'm sure staying on-site at the resort is fun and at least a bit more convenient.
4) The things that are a must-do at DLR will depend, again, on what your children are going to want to do most of the time. With 3 kids, you likely are not going to want to spend the majority of your day taking turns with your husband so each of you can ride rides the kids don't want to go on, while the other one waits with the kids at the exit. As a Mom of 4, I can say that gets old pretty fast, lol. A few times a day is fine, but anything more than that starts to take away from the fun of being at Disneyland, at least for me. If you let us know the temperament of your children (are they fearful of most rides or will they ride almost anything?), and how willing/unwilling you and your husband are to use the Parent Swap system in order to ride rides that the kids can't/won't ride, then this question will be much easier to answer.
