I had this issue with my sister just a couple of days ago. My nephew is a large 5yr old and she is still putting him in a carseat designed for babies. He is way too big for it and the straps have to be so loose just to fit over his shoulders. It is just plain dangerous.

In my case I said to her "wow Beau is getting so big that seat is way too small for him now, it's dangerous". I then said that he should be in a big car seat or a booster seat and got talking to her about the new laws. I offered her one of the booster seats I keep at mums (still not sure if she took it though) and when I saw her a couple of days later on christmas eve, I gave her and mum some brochures on the new laws (my parents had been asking about the new laws too). If I had of known about my nephews car seat situation sooner, I would have bought him a new booster for christmas. As it is I am going to be keeping an eye out for when the boosters are on sale again and pick one up for him as a present. (she will be more likely to use it if I can present a 'new' one as a present, rather then just giving her one of my old ones.) Truthfully I am surprised she reacted as well as she did: she has still too actually act on the infor I have given her, but I was expecting her to completely shoot me down, instead she actually seemed to listen and I know she read the brochure.