Though the "children" part shouldn't really be considered an insult, seeing as this game DOES cater to the age group of children (the majority of League of Legends players are apparently between the ages of 10-17, which by American standards at least, is considered a child), it can be seen as an insult depending on context.
The majority of this game is definitely on the younger side. Can I say that a good portion of the game is probably at the young adult level? Absolutely. However, it doesn't change that the majority of the player base is between the ages of 10-17, ergo, using "children" in this context is definitely correct. The only criticism I could think of is that the people doing the flaming may be a bit older, but flaming someone or insulting someone (for example, in my last ARAM game, the Shyvana on the enemy team decided to end the game by calling us all "fucking idiots" for absolutely no reason) is definitely a sign of immaturity and childishness. I am guilty of doing this from time to time, but even then, that is extremely rare (read: maybe 1 game out of 100 now) for me now in comparison to my old self from 2011 when I first started this game.
(I am not trying to take sides in this post either, but I do want to go a little bit more in depth about my interpretation of it)
@OP: One of the tips that shows up in the loading screen is that the more someone flames or insults their teammates, the worse their teammate will perform, and the higher chance that the person doing the flaming or insulting will lose. Though, from what I've seen from even my own friends, that tip is as often read as the TOS that people have to read before making an account on any game (Btw, most of my friends are in the age range of 23-30, so we aren't really considered children. Though, my friends and I are also just as irked about how toxic the League community is).
