I think it's a reasonable goal to plan for every flight (except on very cheap fares that don't accrue FF miles) to credit miles to some program. You basically need to belong to one program in each alliance and use an appropriate FF account number on all bookings. However, within that, you can plan to pick a FF program in each alliance such that your miles won't expire (or can be kept active easily and cheaply) and will count toward lifetime status. Note, however, that some programs only count status miles earned on that particular carries (and sometimes especially close partners, such as JV partners or codeshare flights) for lifetime status. From here, you can still choose to fly convenient airlines/schedules that offer good fares for your trips.

For the OP's case, note that some airlines do not allow children to have FF accounts while others encourage it (although lap child fares won't earn miles).