Is there room around your spare to stow fluids? Mine is a station wagon and that's where I keep them.
I keep mine pretty stocked. In the back hatch I have a tarp laid out, with a folded sleeping bag and folded towels and a dog blanket on top. Makes it comfy for the dogs and we'll have blankets in an emergency.
Then I have a 72 hour kit in a backpack. There's food for me and the kids, along with those huge natural balance dog food chubs. Lighters, a can opener, knife, forks, soap and washcloths. Deodorant, toothpaste, and several spare toothbrushes. TOILET PAPER. Girly hygiene things (they make an excellent sterile bandage in a pinch)
A change of clothes and dry shoes for regular passengers.
A few gallons of water.
Dog water bowls.
Candles, one of those flashlights you shake to charge up.
Diapers, because of the baby.
And most essential of all, a machete. Just in case.
ETA: bungee cords, a bunch of climbing rope, and some heavy duty carabiners. They're all stowed under the tarp with the machete.
ETAETA: plus a first aid kit and a tool kit.