Prevailing guidance would be to pick the one airline/alliance that works best for your travel needs and funnel as much of your flying on that as you can. Newer thinking is that since airlines seem to be devaluing FF programs annually, the value of status is continually being diluted. Since the time horizon for your children to hit LT status is multiple decades out, you may want to reconsider this goal and instead of being loyal to one airline/alliance just book whatever flights are cheapest/most convenient.