Why didn't Attitcus tell his children that he had to defend Tom Robinson, that he was appointed by the court and didn't really have a choice about taking the case?

Actually Atticus could have refused the case. Judge Taylor knew that Atticus was Tom's only hope. Atticus took the trial because it was the right thing to do. He wanted his kids to know that, even if they were beaten from the beginning, doing the right thing is never done in vain.