I, too, would buy a seat for everyone. It'll be much easier on you all physically. And I second the bulkhead recommendation if you can manage it--there's nothing worse that trying to prevent your toddler from kicking the seat in front of him constantly. Well, there are worse things, but that's pretty miserable for everyone involved.

I also agree on just getting there rather than stopping along the way. It's wearying but best to just soldier through.