Every time you open an ice chest, fridge, freezer you're gonna loose some cold. Having a dual zone fridge is great, but if the freezer side doesn't have it's own access lid, your fridge will have to work harder to keep stuff in the freezer side really cold because every time you open it to access the fridge side, your also opening the freezer side. What ever you end up with, try to load both sides from bottom to top in the order you plan to cook. It's amazing how much cold you will let out "digging through" the fridge/freezer to get what your looking for and then having to put the other items back in before closing the lid again. If you get a dual zone, but not a dual lid, put a few layers of newspaper on top of the items in the freezer side. It will help to keep the cold air from the freezer side escaping while your accessing the fridge side.
If you can afford it, I'd get the biggest one you can fit in your vehicle. Your kids will eat more as they grow and they may want to bring a friend along at some point. It also gives you a little extra room to throw a handful of frozen water bottles in with the food items at the beginning of your trip. This really helps the fridge/freezer maintain the cold temps while drawing less power and once the bottles have thawed out you can drink them, re-freeze them, or move them to an ice chest.
Another thing to consider is an insulating blanket/cover for your fridge/freezer. They actually seem to work well enough to justify the investment. Also, pay attention to where in the vehicle you store the fridge/freezer. I know space can be limited, but keeping it out of the sun and in a space that allows for adequate ventilation will also make a big difference.
I know you say you'd like to get rid of the ice chest, and I understand space can be an issue, but over the years, we've found that having an ice chest (even a small one that can be re-stocked daily) for drinks keeps the kids from standing there with the door/lid to the fridge open while trying to decide what they want. We've also found that drinks in an ice chest will remain quite cold for quite some time if you don't dump out the water from the melting ice. We leave the water, and if necessary, just add a little ice as needed. This works extremely well for us in Baja where daytime temps can get quite high and ice can get hard to come by.
