I think "threw a warhammer at a child" is the phrase you're looking for here. Because that's the in-character event that happened.
Combat rules are screwy and limited. If he was an expert hammer thrower I could see a situation where I would, as the DM say, "You want to use your hammer to 'subdue' the child? Sure. Here's the penalty to hit. If you do hit, and do enough damage to 'Take Out' the child, we can say that you throw your hammer in such a way that the kid trips over it and you can run up and grab him."
The description of how you take out the kid out is fluff.
I might also determine a consequence for failure. In the situation Oofta is describing, the DM didn't do that. Oofta also said that his character wouldn't have done it if there was a risk to the child. The DM was within his rights to say, "You can't do that without hurting the kid."
In the end, can a person make a poor judgment call and in the heat of the moment, do something with good intention to only, after the fact, realize it didn't turn out the way they'd thought it would? Can they do that without it affecting their alignment? People make bad judgement calls all the time. It doesn't mean they're bad people.
What would a character do if they caught a poor street kid stealing food from a merchant in the city? The child is obviously quite poor impoverished and in poor health, so they are likely stealing to survive or to feed their family. The city most certainly has laws that could be harsh for the child now that they are caught, in that it could either be imprisonment or the child loses a hand. What would your character do in this situation now that they caught this little thief?
I think the answer is very obvious for what a NG or CG character would do, in that they'd at least let the child go. But for LG characters this might be more of an internal conflict to them.
I'm not sure if this answers the question at all. I don't really play my characters with alignment.
Who is passing judgement on the character's actions? Is it the DM? Is it the player? Is it the NPCs? Is it the table?
If I say my character stops the kid, reprimands him but then pays the shop-keeper, who is judging my character's alignment? My character might feel they did a good act by being merciful or they might kick themselves for, impulsively, being soft. Or they might be doing a merciful act as a show to make themselves look better in other's eyes.
The same action but, potentially, different alignments.