Also, something that would be insignificant for an adult in terms of how long it would take to grow out could be endless for a child, because sometimes their hair takes a while to get the whole "growth" message. My 6-year-old has shoulder-length hair. It's only been trimmed twice in her life, and just a small amount each time. It's just SLOW to grow. She was bald until 2. We decided to grow her bangs out at age 4, and it took 2 years. That's two years of her hair in her face, her chewing on her hair because it keeps going in her mouth and then the hair gets wet and gross. Trying to pull her bangs away from her face. Taking 10 minutes instead of 2 to figure out some way to keep her hair back for gymnastics (and having it come loose and look a wreck every time anyway). She *finally* in the past month or so appears to have reached the length where it at least sometimes stays tucked behind her ears and it's too long to naturally go into her mouth. If somebody decided to cut bangs for her again... I would be really, truly, awfully angry. They would be costing me hours and hours and hours and hours and hours of frustrating work, working with my daughter to stop chewing on her bangs when they reached that length again in growing out, or possibly having bangs again, which I'd have to keep trimmed and deal with constantly, etc. Hair is not something that you necessarily recover from easily.
Even my second daughter, whose hair grows at a more normal rate, took a while to grow out of a bad haircut. She once took a pair of scissors to her hair and cut her bangs a bit farther along on one side. It wasn't obvious at first, because it was the same length as her bangs and disguised by the other hair. But as her hair grew out, as my husband said, it looked like she had a mullet. She still has a chunk of hair shorter than the rest, because her hair is long and she wants to grow it long like "Rapunzel," so I hate to cut six inches off all of her hair just to make it the same length as that chunk so it all matches.
So having somebody cut hair that you're trying to grow out can be a pain. Even if you *were* going to cut the bangs and they just did it wrong, it might take a few weeks to even out. But especially when you're trying to grow hair out or change the style, you could be costing them months of extra work and frustration. Or years, in my case.