Nephew won't keep chest clip on for MIL--WWYD?

[McGucks]

MIL mentioned last night that our nephew (4 years old) can easily unbuckle the chest strap on his carseat.  She said that she made him keep it on when they are driving on the interstate but that she "lets it go" otherwise.  I am pretty sure that SIL and BIL don't know.  I know how dangerous this is, believe me.  I do not want to get MIL in trouble with SIL (they have a difficult relationship, as do SIL and I), but I also don't want to be complicit in my nephew's injury or death if there were an accident.  I will add that the nephew is pretty strong-willed and I can see MIL choosing her battles with him, but IMO this is a big battle to choose, whether it's on the interstate or just down a country road. 

Is there a product out there that could be used to guard the chest strap?  WWYD?

Should I cross-post in Family Safety?

[limabean]

He probably does it with his carseat buckle in his parents' car too. If I were your MIL I'd just mention that he's doing it and ask the parents if they've found any tricks that work to stop him from doing it. 

[nwatt]

Duct Tape

[batsister]

What about the old trick of pulling over and stopping the car until he complies?  I don't know if it would work on a 4-year-old, but that's how my mom got resistant friends of mine to keep their seatbelts on when we were kids.

[Drummer's Wife]

Can they turn the chest clip around and tighten the straps?

[octobermom]

Well truthfully the cheast clip isn't a necessity. Its just to help position the harness straps and is designed to breakaway in th event of a crash anyways.

however the issue with undoing his is hes now pushing the harness straps outta wack and they may no longeer be fitting the child correctly. Same with sliding the slip up and down.. Ive heard some putting velcro over it the feel deters them..