When you're young, you can go anywhere, anytime, and either just hang out or go hang-gliding or anything else ending in "ing" that involves putting your life on the line. You're free of responsibilities and, as you think about the world over a couple of beers as night draws in, it never really occurs to you that having children is a life-changing event. Only when confronted by the practical reality of two toddlers does it occur to you that hang-gliding with them suspended from your belt might not be a good idea.
So the burning question is where do you go and what do you do when you get there? Let's start with a fairly obvious problem to avoid. If you've been carefully training these little ones to sleep when it gets dark, going too far north into Alaska is a bad idea since the sun tends to stay in the sky more hours than you like. Stay in the southern regions where it goes dark at a sensible time. Now think about how much kit you need to keep the toddlers clean and fed during the driving. In addition to the seating to keep them safe, you need easy access to all the stored essentials without having to stop and dive into a crowded trunk every time you need something. Now think about breast-feeding. You need adequate room and it might be a good idea to have curtains at the windows in the back of the vehicle.
Do a strategic run-through in your own vehicle to get an idea of how much space you're going to need and then check on what makes and models will give you what you need with a miles-per-gallon rate that won't break the bank. Once you have the field narrowed down, it's on to the internet to find one of your preferred makes and models from car rental companies at a price you can afford. That's potentially going to need some negotiation with the car rental companies to get child-friendly seats of the right size fixed in place ready for you.