My mum occasionally looks after DS (once in a blue moon really) but have just found out that she hasn't been pulling the car seat straps tight enough.
( I pull them so they're pretty tight and can just slip a couple of fingers under).
She refuses to do it, she doesn't think he's be able to wriggle out and thinks I pull the straps far too tight.
I'm his Mum, as far as I'm concerned what I say goes, and she should bloody well respect that!!
(yes, and I know how lucky I am to have a Mum who'll look after DS sometimes...)