Yes and no — there are basic rules with basic margins of error — but as you point out, there’s also political forces that have nothing to do with safety.

In a straight flat road with no other vulnerable road users, the safety margin between 40 and 60 in a modern car is negligible. But on that same road the margin between 60 and 80 is not.

And places change — what was once an isolated country road can easily become a suburban junction. Your example of a child getting hit can either be political reaction, or a lagging indicator that children have become more common in that place.

Edit: the counter point to “oh the state just wants money” is that the state has to pay for roads, and people hate paying taxes. Maybe instead of playing games with fines, voters should just pay for the public services they want? That is part of the reason enforcement is not more serious in many places — it’s not be because voters care about safety — they just don’t like paying fines for rules they chose to break.