Hey - externships were extremely important both from an interview perspective and for your own benefit so you know what you want. I did 3 total and I felt that was a sweet spot. A few of the places mentioned they did not extend many interview offers to people who did not extern (For example, at Boston children's, out of the 20 interviews last year, I believe 18-19 externed). I think it shows interest in the program and also really helps you decide if you like that program. Part of residency is finding the best fit, not just matching somewhere, so I think externing helps you figure out what you like and what you don't. Just so you know, a lot of programs understand that travel/time off in dental school can make externships difficult so I wouldn't kill yourself trying to do a ton of them but I would definitely recommend going to visit your top 1-2 choices. Do it in August when the new residents are there so you can meet them (as they will be your co-residents next year) and at a lot of places they have say in who gets interviews.
I came fresh out of dental school and matched at my top choice; I had friends who were early 30's, worked a few careers (not even pedo related) and matched at their top choice so your second question is a bit difficult to answer. That's probably a program by program kind of thing; I can't see how pediatric experience would be negative in any way though.
Another question I got inboxed but might help some people out:
"how many programs did you apply to?"
I think the average I heard was around 7-10 for most people. I think some people overshot it and did 15+ because they weren't sure if they'd get any invites. You'll run into people that only applied to 2-3 because of location/family so it all really depends. If I was doing it all over again, I think I'd aim for about 10 if I thought I was a weaker candidate, and around 6-7 if I thought I was a strong candidate. Just know some of their interview dates will coincide so if you get a couple invites you might have to skip 1-2 just because they are the same date as others.