We just were at Disneyland with our grandkids (ages 5, 7 9, 13). We got there as soon as the park opened which is around 8 am ish. It takes awhile to drive into the humongous car park, take the tram, walk to gate, stand in line, etc.

We did the California Adventure in the morning and Disneyland in the afternoon. We didn't do everything but we did a lot. But we also live within an 1 1/2 drive, so we can go whenever plus we've been many times.

If one has never been a whole day and evening at either just one of the parks would be plenty of time to do most of the fun rides.

We went during spring break and it was packed. If you are able to go off season it will be much easier.

With my kids being 4 and 2, do you think I would need the Fastpass or MaxPass? I think we will most likely be restricted to Fantasyland due to the kids not being old enough for the other bigger rides.

https://disneyland.disney.go.com/guest-services/fastpass

This is a list of the Disneyland FastPass rides./

Just make sure you give yourselves ample time to enjoy things. Don't rush from ride to ride, trying to jam everything you possibly can into one trip. That makes for tired and stressed people.