I know the carseat thing has been done loads, but I have a dilemma.
I'm a childminder. One of my mindees is 5 months, and I guess is a little big for his age, but nothing out of the ordinary. Yesterday when his mum dropped him off she had a stage 2 forward facing carseat with her. She was very proud of it and told me how it was in the sale so they decided to get it a little early "as he's about the size of a 9 month old anyway" - He isn't, but IMO thats not the point.
I also had a 3 year old with me yesterday and had planned a trip to a farm, about 45 minutes drive. I couldn't bring myself to use the seat, so used a spare infant seat that I had. Usually I think, your child, your choice, but this was in my car, while he was in my care. I'm not sure what to do next time. I only have him once a week.