it sounds like your mind is made up on this, so I'm not sure what it is that you're asking. The "finer points" of hailing a cab? Do you want to know what's easier? To me, arranging for Belize Transfers to pick them up at the bus station and drop them off at the airport sounds at least as easy as trying to get a cab at the bus station and then find another cab at the border. Why can't you get a bus schedule? The ADO bus schedule
is on the web, although the website is often temporarily off-line. One they get on the bus they have about six hours to phone a transfer agent and tell them when the bus is scheduled to get to Chetumal. Like I said, it seems like your mind is made up-I just don't completely understand your reasoning.
Now, if the kids have at least fair spanish language skills or they can find some other way to communicate to the cab driver, they should be able to get to the border crossing without any problem. Once through customs and immigration, getting a cab to the airstrip in Corazol Town shouldn't be a big deal, either. But when I travel I like to keep the "should be's" to a bare minimum, especially when I'm on a new route. Good luck to you.