A wonderful thing that you've done for your daughter...
About triathlons: I did an Ironman in 1999 and a bunch of triathlons over a couple of decades, and yet feel that Nostatic's advice is wise. Aim for health, not for position. My goal for the Ironman was to finish without losing my job, my wife or my health. That's harder than it sounds.
I'm also the same age as you and am already humbled by my Jr. High son's cross country team speed
so I know how easy it is to fall behind physically as your focus turns to raising your children.
That said, if you're a strong swimmer, you're already in a good starting place. The real trick if you're a strong swimmer and yet haven't done a triathlon before is to build up biking/running (together/back to back in a "brick"). No matter what shape you're in, you won't believe how poorly your legs feel when you jump off the bike and start running. Practicing gets you used to that feeling so you can more easily work through it. If the run is 3 miles, expect most of the first mile to feel very rubbery.
Rest is crucial at this age too, so do one long day a week and 1-2 shorter workouts in each of the sports per week. Take 2-3 full days off every week so your body can recover.
Good luck & enjoy! It is a great sport!