Interesting question, my DC are late teens/early twenties now.
I wish I'd taken DD out of school and homeschooled when she was relentlessly bullied and school didn't do much to support her SEN. I feel guilty about it but being disabled I know I wouldn't have managed it anyway and to an extent it gave her life skills. It just gave her some real issues too.
Like others I'm glad mine grew up without smartphones until they got into their teens. I think parenting has massively changed in the last few years and feel sorry for those navigating all that with youngsters now. I still think I was a bit lax with screen time and gaming etc but it was ds' hobby and he still loves it and is a lovely, helpful, caring young man, so it can't have done too much harm.
I wish I'd taken hold of the moments more when they were little, and not always been so rushed to get them into bed like pp say. We did read to them every night and they both adored reading as kids, now DD still does but ds rarely picks up a book.
Mostly I don't look back and think I wish I'd done it differently because they're lovely people and great company, very opinionated and confident in that which we brought them up to be I guess (it can be wearying at times when they want to discuss politics at 11pm and argue with you again about culture issues). I think we did ok in general, yes we made mistakes for sure, and there are some moments I cringe at looking back, but in general we muddled through and that's enough. When mine were tiny it was all controlled crying, I didn't like extreme versions of that so did gentle versions like the baby whisperer and I don't regret that because they were both great sleepers and happier babies for it, and always confident that they are loved and valued. So I guess I'd say to parents now, don't sweat that small stuff too much, it's ok if you make a few mistakes, it's ok to do things differently to others as long as you're giving that strong basis of unconditional love and gentle fair discipline.