Hey Amanda,
As far as trade in values go, I am not to sure. When I was looking to find my Denali, I went to
http://www.nadaguides.com for values.
As for questions on vehicle matience,
http://www.edmunds.com has comprehensive lists of common things that go wrong. I love my 2003 Yukon Denali.
Not sure about the kids seat, but I believe I have seen threads and it does fit, not sure though.
Good luck in your search