If you think this is so easy and obvious, please provide specific details for this law. For example, what age constitutes a child for the purposes of this law? What, specifically, constitutes missing? How long and under what circumstances, e.g., since you last saw him face to face, since you last had any contact, since you expected her to return? What if you have reason to think the child is with someone else, e.g., your partner or ex-partner, at a friend's house, on a trip, etc.? What extenuating circumstances are acceptable, e.g., the child has a history of disappearing for a couple of days to visit grandma, or your ex, or that boyfriend you forbid her to see? Do you call the police anyway every time, wasting their time on known false alarms?
The devil is in the details. The immediate issue I see with any law like this, especially one jammed through, is that it will fail to consider all sorts of real-world situations causing a lot of innocent parents to be unreasonably harassed when their children aren't actually in any danger whatsoever. But I know, Think of the Children (tm)!