There are four sons in the hagada. How come there is no son called "tzadik"?? Maybe the chochom should be instead called "tzaddik"? That would be the opposite of " ben ha-rasha"!
One answer may be that ALL of the children are tzadikim - "ועמך כולם צדיקים". Every Jew is a tzadik [even though he might not act like one]. Even the "ben ha-rasha" is a tzadik. He is a mixture of tzadik and rasha. If we would call one son "tzadik" that would imply that the other children are not tzadikim. We have to make all of our children feel that they are tzadikim.
Another possible answer - They are just children. How can a child be a tzadik? He has so much to work on. A tzadik in kabala is a "shomer bris". A child doesn't have tests with the bris like an adult has and thus cannot yet actualize his tzidkus. A tzadik is constantly waging war with his bad middos. A child has a looong way to go until he becomes a tzadik in practice.
לזכות ר' רפאל משה בן אסתר זיסל הוא וכל אשר לו לברכה והצלחה