You cannot legally get your children's college grades without their explicit permission. FERPA (Family Educational Rights and Privacy Act) in the United States protects student educational records. Once a student reaches the age of 18 or attends a college, they are considered an adult and have the right to control access to their own educational records. This means even parents who paid for their child's education cannot access their grades without the student's consent.
The only way to obtain your child's college grades is if they choose to share them with you.