Can we legally sit two adults and 2 kids in the backseat of a 2006

I believe you need to each have a seat of your own. I know when I was younger we'd fit 6 people in a regular car, one sitting on the backseat floor, but it can be too risky considering you can get pulled over and you could always get into a car accident no matter how good of a driver you have controlling the car.

Well, are there four seat belts in the back? No.

So, no. That would be illegal.

no absolutely not each person must have on their own seat belt while in the auto and during the entire time auto is moving. you may want to try a van.