Convertible carseats? I need your input ladies!?

I don't know what their car seats are but my kids are 3.5 and weighs 38 lbs and the other is almost 5 and weighs 41 lbs. They are both still sitting in there car seats. It depends on there body size. My daughter isn't fat at all but she is very solid as for my son he isn't scrawny but he is petite. She will be out of her seat before him.