I have seen a mum arriving at school more than once with her kids not belted in in the back seat (they are 5 and 3). I also think she is not using the seat belt for her baby, but putting the seat on the front seat. It doesn't have a belt round it, but could it be on an isofix in the front passenger seat? I didn't have isofix in my car, not sure how it works.
WWYD? It might be that the kids are just undoing their seatbelts as they arrive, the baby might be isofixed. I don't want to get all judgey and bossy, laying down the law to her, cos who made me boss of the school run? She's not from this country but her country (EU) has the same laws as UK, but there is a language barrier to get over too. Do I butt out or wade in??