Geektastic:
It's a bit like drafting a Trust Deed.
The Trustees will usually be placed in the position of having to use their best endeavours to determine some aspect of the Settlor's intent. The courts would usually only expect people in those positions to do the best they can. As long as you've taken professional advice, not acted in contravention of it and the trustees (or, in this case, executors) have discussed it and agreed a course of action that the man on the Clapham Omnibus would not consider unreasonable it is unlikely the court would consider that you should have done more (what more can you do, for a start?) although I guess you never know with the law.
If it were me, I would personally define benefit in this context as being something that has lasting positive outcomes for the beneficiaries. I'd say that within that sort of area you as executors are actually entitled to use your judgement and that the person who made the will intended you to do so because otherwise they would have been absolutely specific and left it (for example) as education and maintenance.
Thats where we see it. When they are due, they make whatever choice they like. When they are not due, but request funds, the bolded must apply, as distinct from cool, I want this I want that. Thats why the age is 26