There's nothing inherently wrong with involving donors from the general public in such an appeal, but rather than stating that "The Dahl family has already given generously to save the writing shed. We're now going to trusts and foundations and to the general public" as museum director Amelia Foster did, I would have much preferred a scenario like this:
"The Dahl family has made a gift of £250,000 to save Roald's shed, and we invite his fans and members of the public to join Sophie, Liccy, and the rest of the Dahl family in securing the final £250,000 we need."
I know that some people will balk at the idea of a public appeal when the Dahls could pay for the entire project themselves, but others will have a desire, a need, to be involved and they should be given the chance to participate.
Rick Holland CFRE MInstF