I Found a Unicorn!
Not really but I have found a truly spill-free, easy-to-clean cup which is arguably just as rare. May I present the Contigo Kids Tumbler.

I/my children have dropped them off the table, left them on their side, and held them upside down . . . not a drop comes out! The straw twists apart for easy cleaning and they have held up through repeated dishwasher cycles. The one negative is they are not cheap, a three pack on Amazon costs $34, but the saved sanity and laundry makes them worth every penny!