https://secure.ssa.gov/apps10/poms.nsf/lnx/0200602020 Read A.3 and B.3

Children's benefits are not designed to be for the family, but can be used for a reasonable share of household expenses which does benefit the family.

That is not the same as using child's benefits to buy a car for mom or dad or to mom or dad to take a trip. The OP was talking about paying for a trip for the child from the child's benefits which obviously benefits the child. But it doesn't mean that the lump sum should be used for the family to take a trip.

This is true whether it is disability auxilliary benefits, retirement auxilliary benefits, survivor benefits or SSI. SSI has even more stringent rules on the spending of the retroactive benefits if the back pay is more than six months, but that is not the question being asked here.