Ugh, I remember that! Quite a predicament! My daughter hadn't ever done that, and then one day I was driving down the road and looked in my rear view mirror and saw a CLOSE-UP of her! She had gotten all the way out of her seat and was standing in the center of the back seat poking her head through the two front seats! You can imagine the shock! Luckily I was still in my neighborhood, she'd done it immediatly, so it was easy enough to pull right over and clarify that "that was BAD", and I went back home and grabbed one of those rock-climbing hooks from the garage to clasp around it. Then I wound up getting another car seat for her that was more complicated to open, as hers just had one button to push then you pull it apart. Some are much easier than others. I think we as parents tend to like the easier ones, because it's easier for us to get them in/out, but over time, it becomes easy enough for the child, and that is where the problem lies. I found one that has a mechanism that is a bit harder (it was a pain for me too), where you have to push two thingies at once to get it to come open, and there are several other seats that make it a little more complicated. Buying a bunch of different car seats isn't much of an option, but perhaps buy one, and keep the box and receipt handy, and if she figures it out within the month then take it back. But do set rules on that now. Because before long she'll be in a normal seat belt - and we all have to follow the same rules in life. We can't just take our seat belt off when we feel like it. She won't be allowed to do it when she's older either, so go ahead and be setting the ground rules now. Be sure there is a consequence for each time she does it, and be sure she understands the consequence. Rewards programs work great on many kids. We normally eat quite healthy and low sugar, so treats are a big thing in my household. I would keep a bag of marshmallows in the car and after every ride give her a marshmallow if she did great on the car ride. Every time you get in the car and put her in her seat, remind her that if she doesn't un-do her seatbelt, that she'll be getting a treat. Ask her "What happens if you take off your seatbelt?" and be sure she knows she doesn't get a treat. My daughter will do back flips for marshmallows, but you may need to choose a treat that you kid is coo-coo for and doesn't normally get. Those are my suggestions, hope they help. I do think the time-out thing is a good one if you can make the time for it. Perhaps stage short drives around your neighborhood and in places that you know you can easily stop and pull over quickly, of course she may not EVER do it then, just when you're on a major highway or in a big hurry! :o) But pulling over on major roads can be extremely dangreous too. So pulling over all the time to discipline her on something that is dangerous (as you're doign something dangerous), just doesn't make a whole lot of sense to me. Your 5 year old may be able to be of some help. Explain the situation to him and ask his advice. Seriously, sometimes kids can be real smart! If nothing else, he can alert you when she is messing with it so you have a little advance warning... I believe the key is in finding the proper way to discipline her on this, but in the meantime, do whatever you need to do to be sure she's safe. The seat belt tightener thing was a good idea someone gave. So, Mom, fix the problem for now, and then focus on correcting the problem within the child. Some kids are easy to discipline and don't need much of it, some are just relentless. So it's easy for us moms to tell you what you're doing wrong or right, but you're the one dealing with your child, so only you can figure out what works best for you & her.