Can you give me any advice on trading in my cars?

KBB is a guide, and typically a dealer will try to negotiate a lower trade in value because it will increase his gross profit. Sell both at private party average retail, pay off the loans and use the proceeds to make a down payment, don't trade in to the dealer because the remaining balance owed will be added to your new loan and your payments will be higher. Being "upside down" in a car note is a bad thing obviously.