Cate likes to sit in chairs. (Or couches + benches, which she also calls chairs.) I guess she decided Norah's car seat looked like good-sized chair for her. So she sat in it. Then Luke decided sitting in Norah's "chair" must be fun, so he beat Cate to it after she got up. Cate wanted back in, so she just sat on Luke's lap!
Yes, Cate saw me with the camera, so she's saying, "Cheese."