Not sure how far you are from BWI, but Southwest has direct flight from there to OKC at a reasonable price, esp. if you get them on one of their sales. Maybe you could look into that and see if the price would be acceptable to dad.I am the obligee. Dad is the one who pays child support. It is about 1400 miles from va to ok. Children ages 13 and 10 and trains are less expensive and not an option I've looked into. Greyhound is a big no as they do not allow kids this young to travel alone at least that what their dad has said as he looked into that option.
It might be a bit of a trip to the airport for you, but best for the kids.
Just an option!