Maybe they did. The article didn't say. All it said was that the chartered bus ran into heavy traffic. I have had many occasions where planning to arrive 1/2 hour early barely got me there in time (best laid plans and all).
Luck is an element of every outcome. The more you care about a particular event, the more you factor in the possibility that everything will go against you. The White House did these little kids a favor by refusing to see them. It was not the kids’ fault they were late, but that’s what is valuable for them to learn: just because something isn’t your fault, doesn’t mean the world will adjust for your bad luck.