If you achieve gold status, your children can have FC accounts. However, the miles they earn are credited to you.
Reward availability is generally good, in my view. Just now I experimented and found there were lots of days in May when you could get 4 UC rewards to Orlando from LGW, for instance - so you do have a chance even on the LGW routes which have a small UC cabin. Out of LHR it should be relatively easy to find 4 seats to US destinations. HKG and SYD are more tricky.
The key to reward success is persistence, as VS release the seats over a long period.
It is of course true that compared to the BA MFU from W to J, the VS scheme might seem a bit less attractive, but the ability to upgrade from Y to J can be a godsend - especially if your Y travel is paid for by your employer.