Is it immoral to have children? (...) A moral system for human beings needs to (...).

All hail the unified global moral system. What a dystopia that would be.

Keeping in mind both the cost and that on average people (...)

Well, that's not the reference group you'd want to be looking at (thus weak evidence).

(...) our influence on the adult behavior of our children is not that big.

You provide both nature and nurture, and it's up to you to avoid the usual pitfalls. There were some recent posts on LW on how to raise critical thinkers. Now, intent doesn't equal intended effect, but a large disparity in effect size -- relative to "the norm" -- is to be expected.