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If they're flying into Love Field, there's a short bus ride that goes to the Green Line and is still about 3 hours total.
Also get the DART Regional day pass. There should be a ticket machine at the rail station or download the DART app. Regional passes work on Dallas, Denton, and Fort Worth public transit systems. $10 for a regional day pass last I checked.
The GoPass app is how you use a credit card.
Android - https://play.google.com/store/apps/details?id=dk.unwire.projects.dart
IOS - https://itunes.apple.com/us/app/gopass/id673970506?&mt=8