Edit: I guess I didn't word this very well. Please consider this comment in the context of my earlier one: https://news.ycombinator.com/item?id=34901931.
One major problem, at least from my perspective, is managing the transfer of responsibility as a child progresses from toddler to adult. It's not like you throw them the keys at 18 and say 'good luck!'. So there is ongoing give-and-take, rules, negotiations, and extending trust. My kids are 10 and 12, which is an exciting time for sure. Puberty is a wild ride whether you're the victim or not :)
Haven't let her get a smartphone yet, but this is an ongoing battle. Because when she says 'but all my friends do!' she's not lying. I have to temper my fears and try to remember what it's like to be a kid. As an adult I had largely forgotten what it was like in middle school.
> Parenting is by far the most difficult, stressful thing I've ever undertaken.
From my observations, I don't think this can be overstated, at least from an American perspective. It is easily the single largest factor in my decision-making on this topic, and seems to be something which many people underestimate.