I had to approach a situation with my sister who put my nephew in a booster seat too early. It was a really, really touchy situation. She and I have been working on our relationship and it'd been getting better, but I knew I couldn't say nothing about the situation. Her ped had told her it was okay, but seeing him in that seat- there was no way he should've been in it (> 4 yrs, > 40 lbs). Anyway, I ended up getting my mom involved and she ended up taking my sister to get a new seat for my nephew. My sister wasn't happy, but I was going to buy her one and ship her one if my mom didn't handle it. Because if something had happened to him, I wouldn't have been able to forgive myself for not... But, it wasn't an easy situation. GL! (FWIW, everyone's fine now- it died down after about a week & my nephew's safe. Well worth it!)
I would say something. I would try to casually bring it up. For example, say, "Wow, your little Johnnie sure is getting big. I can't believe how much he's grown! How much does he weigh now?" When the mom/dad answers, say, "Oh, shouldn't he be in a different carseat now? I thought the weight limit was xx pounds. You might want to check the manual on that."
If it was my neice and I was absolutely sure the carseat was unsafe, I'd say something. But I probably wouldn't say something to anyone else. Maybe in casual conversation I might mention the difficulty we had finding the right seat for Gracie when she was outgrowing her seat.
Re: If you knew someone was using a carseat past the weight guidelines...
I had to approach a situation with my sister who put my nephew in a booster seat too early. It was a really, really touchy situation. She and I have been working on our relationship and it'd been getting better, but I knew I couldn't say nothing about the situation. Her ped had told her it was okay, but seeing him in that seat- there was no way he should've been in it (> 4 yrs, > 40 lbs). Anyway, I ended up getting my mom involved and she ended up taking my sister to get a new seat for my nephew. My sister wasn't happy, but I was going to buy her one and ship her one if my mom didn't handle it. Because if something had happened to him, I wouldn't have been able to forgive myself for not... But, it wasn't an easy situation. GL! (FWIW, everyone's fine now- it died down after about a week & my nephew's safe. Well worth it!)
DD, 1/7/05 * DS #1, 1/25/07 * DS #2, 11/11/09
Baby #4, EDD 11/11/12
m/c 7/30/08 at 12 weeks (blighted ovum, emergency D&C)