It's so depressing that people apparently believe a 3-year old (or any young child) sitting placidly through a three-hour film with 'scenes of fairly graphic torture, scenes with limbs torn open and graphic realistic injuries from the beginning, beloved avengers dying in open-eyes, broken necked kind of ways' is a sign that they're brave, smart or in some way advanced.
If anything, the lack of any real reaction to horrible images is the problem, not the proof that everything's fine.
Still - it's obvious that many posters are really invested in being the Cool Parent, the one who isn't bound by all these stuffy rules and who laughs in the face of convention. The weird thing is the perception that this attitude is in any way unique or special. The fact that the cinema is packed with young kids should be enough to suggest that this is a boringly prevalent approach: expose your kids to any old shite, because as long as they're not crying or having nightmares then it must be fine.