It sounds as if your dependents may be listed in a non-dependent status. Revisit My Info (or Personal Info in some versions) to review the status of the dependents. TurboTax will only allow for the selection of the primary or joint taxpayer or another person listed as a dependent.
For college-aged students the most common entry error that will cause them to be listed as a nondependent is the question "How many months did... live with you?" Temporary absences, such as those for school, don't count so be sure to answer "The Whole Year" here.
Below is a screenshot that will show what the different statuses look like when you return to the My Info/Personal Info section to review the status. Only those showing as dependent will show on the 1098-T selection screen.
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