Just saw this on TV the other day, and I've got the same question as well. In fact, I fully expected the other three henchmen to attack when Nick found the kid. I've heard this praised as a great action scene, but compared to the other three other major scenes featuring swordplay, this one was the least of the bunch (but still pretty fun), not coming close to the duel with Sho Kosugi, the scene where Nick takes on those two cops in the house, or when he slaughters all those armed men when the power goes out.
I recall a quick scene immediately following the "window seat" exchange that shows Slag watching Nick and Billy's bus as it pulls out of the terminal, which obviously implies that he's shadowing them.
The remaining bad guys aside, that scene in the corn field was pretty fun to watch. Although I find it hard to believe that a blind man can cut through a piece of wood, which apparently kept the whole shed together..
That's not so hard to figure out. There's a blind man and a little kid, none of which can drive a car (although Nick does that anyway, later in the movie...). So some kind of public transportation seems a logical way to get away. So just ask people at nearby bus and train stations if the have seen a blind man and a little kid. Someone should remember them getting on a bus.