As both a longtime MMO player and occasional LoL player, I can completely understand the frustration and annoyance that comes from a team member suddenly needing to afk. WoW and FF14 raids can go into a complete standstill, and LoL games could be lost. In terms of actual time wasted, it is very much akin to pulling a kid from a rl sports game, with less of the stigma/embarrassment involved for the parent.
I'm also not ok with people saying "it's just a game" because that doesn't make it any less rude to the other people just because you personally won't have to deal with the repercussions of wasting everyone else's time. Online etiquette is different from irl etiquette but it still exists (or should).
THAT BEING SAID, the parents have the final say, period. The online people don't know what's going on, and chances are it was the kid who joined that game knowing it would take too long and knew that they weren't supposed to. Kids mess up, they are kids and human beings, it comes with the package. And they don't get a free pass, the parent has to put the foot down when they mess up and are doing what they shouldn't. And even if the parent is the one being an asshole, they are allowed to be til the kid turns 18 and can gtfo.
With the systems of most MMOs and MOBAs they are likely gonna have to learn to be responsible and manage their time better regarding irl vs gametime for the future if repeats happen (MMO static rep, MOBA leavebuster, ect) and that's actually a really good skill to have. Parents aren't totally blameless in everything they do, but that's ok, this is just a "danger" of playing with more than one person, sometimes stuff happens (ironically it's also why my MMO statics are all 18+ members. We don't hate younger people even a little bit, but realize that they aren't always in control of their life as much as us and stuff...would rather not have to deal with adding angry parents to a laundry list of things that could go wrong)