Honestly, that orientation is USAUALLY loudest on the meter, but with kids, I don't think it's your best option. When the kids or passengers are in the car and the seats are up, your subs and port will be stuffed against the back seats.
Unless you just HAVE to have some super pretty built in install, I'd go with a pair solid 10's or a 12 and just do sub up port to the side in a box that's relatively easily removed in the event that you need the trunk space.
You'd be surprised how loud and how good this seemingly very simple setup will sound even with the seats up. I do suggest removing the rear deck speakers or riding with the rear armrest down if there's a pass behind it.
I rocked a similar setup in my Lexus for like a year. Single SA12 sub up port into the passenger corner and it soudned great on ~ 1k. It did 141-142 in the 35hz range. I started to go bigger, but HATED all the weight back there and the resulting ass sag.
If you compete, you can always have a comp setup ready to go in.