I don't even remember what I did when I first joined aside from maybe hang out in pubbie hubs and camping linden trees for newbies, haha.
I don't have any kids of my own right now, but I will say that my mom introduced me to MMOs when I was about 12 years old and did it in such a way that she was able to teach me how to be safe there and what to avoid. Granted, your typical MMO is a lot more fenced in in terms of content, and SL can really be a wildcard. So, if I had kids, I think I'd try to explain what I was doing/playing, but obviously would follow the guidelines for age restrictions - if you can't play til you're 16, then you can't play til you're 16. At the end of the day SL isn't really a game - it contains games, but it's more of visual platform than anything else. There's so much to do and see still even if a lot of the old hangouts are ghost towns.
TL,DR: If my future kids are old enough to have an account by LL's terms, then sure. Just be careful and don't give out your personal information, and be aware that people can and will lie online about who they are.
Lewis - your experience is super sweet and wholesome and I love that it worked out for you and your daughter that way.