4 y.o. taking off chest clip in car seat--WWYD? Tell parents?

[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. 

Also, there is the possibility that SIL and BIL let him do the same thing.  I have no way of knowing.

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

[Maedze]

Of course you should let them know.  They should be aware your MIL is not correctly restraining their child when she drives. 

However, it might interest you to know that the chest clip is a crash positioning device, not a restraint device.  It is by no means strong enough to restrain a child.  The harness and the crotch buckle are how the child is restrained.

The chest clip is intended to make sure the harness is lying correctly on the child prior to a crash.  If the parents, or your MIL, tighten the harness correctly (and I realize this is highly unlikely as most parents don't bother to correctly tighten the harness), so that there is no slack, and you cannot pinch any harness at the shoulders, the chest clip being unbuckled should not affect the function of a seat in a crash.  

Discipline should be used to prevent the child from mucking with the clip.  No aftermarket product should be engaged.