We took the family to Disneyland six years ago, 2 adults and 2 kids, 11 and 13. We stayed 4 days with Park Hopper to visit both parks (Disneyland and California Adventure), plus attending the late-night Halloween celebration that runs in late October. Analogous to your upcoming trip, but not on all points. We did not stay on the property, nor did we have a meal plan. (A coworker has gone to DW multiple times, staying on the property w/ a meal plan, and liked doing that a lot.) The financial element was different for our visit, so I can't speak to that.
The main advice I'd give is not to try to see it all. There is
so much, and it's tiring even for an adult. And a lot of your time will be spent in line, unfortunately. Try to use the
Fast Pass system as much as possible; it's a real time saver. (An adult can be your team runner, getting the passes for rides you want to go on later in the day.) Try to leave some free time in your schedule, for more leisurely meals or just as breaks between bursts of activity (rides, shows, etc). Many meal plans include a meal with some characters; those are always fun -- you get some time off your feet, and everyone gets some food. All plusses. And the characters are uniformly WONDERFUL!
You are very brave to do this with a 2-year old. They tire easily, don't like waiting, and don't understand what's going on much of the time. I saw several toddlers melt down in our time at the park. You know your child best; plan with that in mind. There may be times when 1-2 adults may need some downtime with the toddler (including a nap, probably, which might be easier if you stay in the park) while an adult or two takes the 8-year old to a ride or show. (This worked well for us. One night, my wife and younger child went to bed while the older child and I went back to watch a fireworks show. It made a special memory for my oldest daughter while the youngest one doesn't regret it in the least; she was dead on her feet. As was my wife...)
I'd say that's my biggest word of advice -- don't over-schedule. If you're always rushing hither and yon, you'll miss opportunities for some spontaneous fun. Take a picture with a character. Let the kids explore Toon Town. (OMG - amazing!) Have a Mickey's head-shaped snack. Take a carriage, train, or monorail ride and see some sights. Watch a parade. You know your kids; you know when they're wearing out or dehydrating. (Be sure to watch for the same signs in the adults!) Be open to changing a plan when necessary.
Go. Have fun. And if you do it sanely, you'll even
want to go back to see all the stuff you didn't see the first time!

My whole family still speaks glowingly about our trip, and we'd
all like to go back (or go to Disney World).