It's a good point and since we note that children MUST by SOLAS law adopted by every country wear a life vest then shouldn't we all?
The big point is to get comfortable life vests for children. The USCG study showed that drownings have decreased with increased comfortability of Children's life vests. i.e. children complained less and thus the partents were able to keep the life vests on children and were not tempted to take them of to shut up little johnny.