what other seats do you suggest? I really think some kind of booster would be good for the 4yo boy, but which ones are good for him as he is tall and skinny. The baby girl is tall for her age but not too tall but very solid, probably about 30 pounds.
Thank you everyone for your suggestions. I am taking this seriously and plan to talk to her sunday about it when I go visit them.
For the 2yo, an Evenflo Chase -- I don't have a specific site recommendation, but it's usually around $60. It also harnesses to 40#, but the harness measures 16.75" instead of 15" for the Cosco seat. The Chase also converts to a booster -- it's not a super booster, but it will do. Ideally, in 2 years or when she outgrows the harness, you can put her in the high-backed Turbobooster and get a cheap backless for the older boy, but the Chase as a booster is *definitely* better than nothing.