To clarify, would she have to fund the HSA? If so, add the wife and child to your free, great insurance. Put the money in some other savings, just as you would any money according to your investment plan---Roth, 529, whatever is next on your list.
If the employer would contribute the full $3500 the answer might be different, but I personally would always go for the excellent insurance. Even a healthy 31 year old can need serious medical care, especially from an accident.