Hi, Gina!
I have a 15-year old, too, so I can understand your concern for needing to take the little ones back to your resort earlier than the older two.
While there are taxis readily available at Walt Disney World parks and resorts, I would not recommend that you direct your older children to them. Catching a taxi, being sure that you are heading to the correct resort, knowing how and what to pay, etc., is a process far more complicated for a teenager than getting onto a WDW Resort bus. Check out the post entitled
Complimentary Resort Transportation for more information.
In addition, keep in mind that buses run from WDW parks to resorts up to one hour after park closing; when there are more Guests in line for a resort bus than a bus can accommodate, additional buses will be sent to the stop until everyone has boarded and gotten "home". No worries. We've never had a problem getting home via WDW Resort transportation at park close.
When my older child (and her friend) are heading back from a WDW park a bit later than the younger one and I, we simply keep in contact via text...frequently. Cast Members will not let Guests stay in the park after park close, so heading to the bus stop at that time will ensure that your older two children will get to your resort safely, via WDW Resort bus (even if the first bus is full.)
Enjoy your upcoming stay, Gina! Be sure to send more questions as you think of them; we love to help!