We live in Central Tx and took a trip to CO a few years ago, looong ride! We had new toys that we put in a bag for each kid and only pulled them out when they started getting antsy and whiney. They were entertained for a while with them. The color wonder stuff would be awesome for little ones, virtually no mess. Candy and junk is ok to have in these situations when nothing else works IMO. DVD player was awesome and there are Redbox things everywhere so you can swap them out when you need to. We stopped at rest stops for lunch and bathroom breaks (you can look up where they are on your route ahead of time) and we made them run around for a little while. We made them race each other and we did some exercises too, even the adults get stiff sitting for so long! For the baby, we didn't put many clothes on because it gets warm sitting in a carseat for a while, so we put his pants on when we got out.
Have fun and drive safe!
I lift my eyes unto the hills, Where does my help come from? My help comes from the Lord, The Maker of Heaven and Earth. And I'll praise You in this storm and I will lift my hands, For You are who You are no matter where I am, And every tear I've cried You hold in Your hand, You never left my side and though my heart is torn, I will praise You in this storm