Relax. If you were in the front seat facing forward and the kids were 5 minutes into their nap, unless they have eyes that can see through the seat, all they can see is your eyes in the rear view mirror or his or your elbow. If you think your kids seeing your elbow is going to scar their psyches . . . ;-)
Kids should see their father and mother kiss and hug and the like. (No, I don't mean see you or him grab what a thong swim suit covers.) A playful pinch or tap on the butt is ok. Your kids need to learn what is normal and good for good married couples to do to show each other they love each other. Too many women were taught by their moms that "good girls" don't do it("play or be playful" with their husbands). Then they wonder why their husbands have wandering eyes.
Read and follow the book, "The Proper Care and Feeding of Husbands." You and he will have a lot more fun and have guidelines from a professional family theripist. (sp?)
Husbands and wives should set the example for their kids. My kids saw me pat or pinch my wife's butt. They saw me kiss her and hug her too. They also saw me give my wife flowers when it wasn't a special holiday, like Valentines day or Mothers Day. They heard me tell her "I love you", even when she wasn't with in earshot. They saw me consistantly open my wife's car door to let her out or help her in. I always offered her my hand. I explained to them that I grew roses because it gave me pleasure twice. Once when I was caring for the roses because I was thinking of my wife while I was doing it. And once when I gave her the roses and saw the smile on her face and the kiss she usually gave me. I write my wife poetry and tell her how much I love her. My kids have read some of it. They knew I loved and respected my wife because I showed them by my actions.
It has paid off. 5 of my kids spouses came to me in private and thanked me for teaching my kids about how to treat thier spouse.
Relax and show your husband you love him. "The smallest good deed is worth more than the grandest good intention." He is driving. If he wants you and your kids are sleeping in the back seat and he grabs you and you are uncomfortable with that, tell him "just a minute." "Want a preview of coming distractions?" (Big smile and a wink) Then flash him. Tell him he has to wait until you get home. He will anticipate and you can wait until you get home and get behind locked doors. You will have told him by your actions that you really love him and you have the locked doors you need.
AND . . . NO, don't do anything that will get you "caught" by your kids. That's neither exciting nor fun.