Childress makes plenty of mistakes, no doubt about that.
He has called terrible challenges.
He has consistently made (statistically) poor play calls on 3rd/4th and short.
On the substitution play you're talking about, I think that Childress believed the Redskins had substituted.
If the offense substitutes then they have to allow the defense to substitute too.
I don't think the Redskins actually substituted anyone on that play though.
It was a mistake.
But everybody makes mistakes.
A few weeks before that game, Hall of Fame coach Joe Gibbs made a pretty bad blunder himself: Redskins/Bills Recap
He called two timeouts in a row to ice the kicker, giving the Bills a 36 yard instead of 51 yard field goal attempt to win.
Nobody is perfect.
Of course, I hope and expect that Childress gets better at some of the in game stuff this year.