I would look at who costs the most for insurance on your car. That should be a factor if the difference is significant, also look at insurance costs one year after passing. Also factor in that your DP needs insurance that is social, domestic and pleasure plus commuting (or business use if he's doing more than driving to/from work). You only need social, domestic and pleasure if you're a SAHM and not working.
Personally, I'd say you should learn, maybe timings might work to drop DP off at work, or pick him up? You would also be able to do the shopping, run errands, ferry the kids around etc. DP is going to drive the car to work, leave it there all day and drive it home. It's not a great amount of use/benefit, when it's sat at his work all day, but the flip side might be offering to drive him to/from work/bus stop/transport hub so he gets some of the benefit too.
Also, I would look up the cost of your DP running a larger car to/from work. Things like tyres get expensive on larger cars and they can guzzle fuel, especially if they're older.
Regarding car seats, you can usually find out on the manufacturer's website if they are compatible with your car. There's often a car guide or just contact them and ask, they might stipulate which seats in your car you can fit it though.