I have 2 kids and claim 1. They are both in college. How do I claim the tuition I’ve paid for my 2nd child?

You can only claim the education credits for yourself and for dependents. If you don't claim one of your children as a dependent then you cannot claim the education credits.

Please see What education tax credits are available? for additional information.

If the other parent is claiming the 2nd child as a dependent, then that parent can claim the education credit. He/she may (and should) count the money you paid, in claiming the credit for himself.  

Although the general rule, in taxes, is that you must be the one making the payment, to get the deduction, there is an exception for education.

Of course, it works in reverse too.  You may count any education payments made by the other parent when claiming the education credit on the student you are claiming as a dependent