Are you being facetious?
If you had bought them a backplate and wing from the beginning all you would need is a different bladder for the doubles. Their muscle memory of where everything is on the harness would be locked in. If, or when, they grow you may (at the most) need a new harness - or leave enough in the first one to adjust for some growth. Simple!
Suggesting people buy new BCD all the time seems like something a dive shop owner or employee would advise to boost the bottom line of said shop.