My two year old has figured out how her carseat works, and likes "practicing" when we're driving. I've tried the stern voice "no!" and explaining it was dangerous, and she keeps doing it.
How did you deal with this?
Oooh, I live in fear of this day. I think aftermarket covers are considered a safety no no. My first thought is to strongly consider some sort of reward/bribery/positive reinforcement thing. And I would talk about how whenever we get in the car mommy and daddy buckle up and then leave the buckle on while the car is driving. And how all of her friends also do the same. Maybe a story about a little snail who feels so safe and snug in her shell, or something. Custom over reason, you know? I don't think 2 year olds can understand "danger" enough for it to be a consistent behavioral modifier, and if it were my kid, just pulling over and saying "The car doesnt drive without buckles" isn't going to mean much to her 2 yr old self. Unfortunately.
DD did unbuckle herself one time when she was about 17mo.... we were driving along and then in the mirror I suddenly saw her little face pop up over the back of her carseat (she was RFing) and she squealed, like, tada, I found you mom and dad! And I literally screamed and we pulled over. I think it was a fluke, like we thought we latched her buckle but didn't do it right. Which is also terrifying.
My DD was doing this with her Graco carseat at around 18 months. The buckles on the Graco chest clips are super easy for them to open. I had to switch her seat. I have her in a Britax since that's what we have for the boys and consistency in installation is a strong consideration. (This was before the 'toxic carseats' thing came out) She can't unclip the Britax.
I'd take her to BabiesRUs or whatever and see which seats she can't unclip.
We've been through this and were able to nip it in the bud. Whenever DD does something that is very dangerous, I give her what I call the freak-out. It's all an act, but if you're good at it, your LO may never do it again. I fake absolute terror at things that are dangerous. If DD were to run out in the street, I completely panic, wide eyes, terrified high-pitched voice in panic, fear written all over my face so she sees that it's frightening and dangerous and that seems to get the point across. Then I hug her and "no! NO! Danger!" etc. like you did. If you have a child that's not prone to freaking out when you freak out, it may work. DD doesn't get upset when I show that kind of fear, but she does take it seriously and never repeats those actions. If something scares Mama, it must really be bad!
And because it's hard to keep them from doing what they really want to do, I have a rule that she's always allowed to unbuckle it herself once we have stopped and the car is turned off. This seems to let her get her fill of wanting to do it.