In the run-up to Pesach, I delivered a shiur on "Pesach: When the Questions Matter More than the Answers" in a few different venues. On a couple of those occasions, as a tangent to the shiur, I asked this question: Why do we have a wise, wicked, simple and unquestioning child in our list of four, but we don't have a tzaddik (righteous child) in the list?
Some suggested that the wise child is righteous, but I disagree.
Some suggested that we cannot ascribe righteousness to our children, but I disagree for two reasons: 1) These are not necessarily small children, and 2) If we cannot ascribe righteousness, how can we ascribe wickedness?
Some suggested that each of these characteristics is only a trait, and not a complete person, and so we cannot discuss a child as righteous, which is a collective trait. I personally agree that these are traits, but I don't see how one could apply this to righteousness and not to wickedness.
At our Seder, I noted that the four children are all presented to us by the Torah in the context of telling our children about our departure from Egypt. The Torah is advising us that we must speak to each of these children in a manner appropriate to them - regardless of their disparate religious personalities.
If so, then perhaps the message of the omission of the tzaddik is this: Each of our children needs the conversation about the Exodus, in a different way. We should see none of them (and none of ourselves, although we may be parents and uncles and aunts and grandparents) as "tzaddik", meaning complete, and being beyond that conversation. The goal is to educate them properly, so that they will grow toward that state, but a parent should never assume that the child is already there. Look for your child's particular need, and address it, to initiate your child into the history and religious identity of our nation.