If her husband is not savvy about the kids- you could try helping him become more savvy- he is their father after all and they are the parents. They decide together what works best for their family. Savvy husband- more supported wife and Mum.
Car seats can be fitted in 2 ways- properly and safely according to instructions or wrongly and unsafe. Check yourself how you SIL is fitting the car seats and steer him in the right direction. Then he can take the children out for a drive to help his wife. If he is in fact fitting the car seats safely you know your daughter's criticism is misplaced. That may or may be not part and parcel of some depressive illness or just her personality.
No fun being old under Andy Burnham .
Fascinating causes of death 1632


