MIL and SIL looked after DD (age 1) for us today. We didn't leave the car seat because they didn't think they'd be going anywhere in the car- they did have the pushchair so they could walk if they needed to go anywhere.
Anyway, they did decide to pop out in the car- so put DD in a high back booster with the car seatbelt around her. SIL says this one is suitable from 9kg up, but I'm pretty sure the only seats suitable from 9kg (ie stage 1) have their own harness of some sort and don't restrain the child using the car seatbelt. They reassured me that she looked fine in it as she's quite big for her age, and she was good and didn't fiddle with the seatbelt or anything.
I felt physically sick when I realised what they'd done and told them not to do that next time. I didn't want to make a fuss because they did do us a big favour and I know they wouldn't have done it on purpose- but still, to tell us that they weren't going in the car, change their mind, and put her in another child's car seat that's not even the right kind is a big deal, right?