Suspension forks on child/youth bicycle for road ringing good? When riding only roads and well maintained trails, does it make any sense to have a suspension fork on a children's bike (10-12 years)?  

My initial gut feeling is to say no as it just adds weight and complexity to the bicycle.  But I don't have any experience to back this up.
The short answer is no. Cheap suspension forks do only add weight and complexity as you say, and give very little actual suspension. My personal experience with cheap forks for adult bikes are that they can take away some road chatter, but with the increased weight and complexity it is hardly worth it.

The only worthwhile reason to have it must be that the kid likes the look and wants to ride the bike more because a suspension fork looks cool.