Is there any space behind the seats at all? Might be able to put things such as sleeping mats there...
mx-5 has 150l of boot space... so 2 large rucksacks-ish.
Trangias are great- and pretty safe with children... but they do take a long time to cook stuff. We have a coleman alpine stove which is gas, but folds up teeny, and fits into our pans, trangia-style... though of course the gas canister doesn't
You can get 3/4 length thermarests which are smaller packed-size- ideal for ds, and adequate for you I'm sure (unless you're 6' of course) There are also backpacking-specific ones, which aren't as heavy or bulky as the normal green ones.
Get the warmest sleeping bags you can so you don't need to take blankets... if you're too warm, you can always open the zip. You can also get silk liners- keeps your sleeping bag clean, extra warm in cold, extra cool in heat, and stuffed into a bag smaller than a malt loaf!
If there will only ever be you and DS... maybe go for a 1 and 1/2 man backpacking tent rather than 2 man...
It will be tight, but only the same as backpacking without a car.