Why didn't Atticus tell his children that the court assigned him to the case? What is he trying to teach them?

Atticus wants his children to understand that even when something is hard, you should do it if you believe in it. 

Scout in particular does not understand why Atticus would defend Tom Robinson when it is so unpopular.  She gets into fights at school and with her cousin, and has to listen to neighbors insulting her.  From her perspective, things would be so much easier if he didn’t have to do it. 

Atticus tells Scout that he couldn’t hold his head up in town or represent the county in the legislature if he did not defend Tom Robinson.  He tries to explain to her that this case is important to him morally and personally. 

 “You mean if you didn’t defend that man, Jem and me wouldn’t have to mind you any more?”

“That’s about right.”

“Why?”

“Because I could never ask you to mind me again. Scout, simply by the nature of the work, every lawyer gets at least one case in his lifetime that affects him personally. This one’s mine, I guess. …” (Ch. 9) 

In truth, Atticus was appointed by Judge Taylor.  He tells his brother Jack this. 

“… You know, I’d hoped to get through life without a case of this kind, but John Taylor pointed at me and said, ‘You’re It.’”

“Let this cup pass from you, eh?”

“Right. But do you think I could face my children otherwise? (Ch. 9) 

Atticus does not tell Scout that he is being forced to take the case because that is not the point.  The point is that he is taking it and plans to defend Tom Robinson to the best of his ability.  He wants her to learn that even if something is impossible, you still have to try. “Simply because we were licked a hundred years before we started is no reason for us not to try to win,” according to Atticus.