JFC some people are being just awful to the OP on here. Awful!
If my (for example) grandmother passed and left me, say, £35,000 and I knew that (1) she was very fond of my DC as “my” DC, so irrespective of whether she knew them or not (2) she hadn’t met by SDC (3) that £35,000 was a lot of money to her that she’d saved over years and at cost to her,
then yes, I would be saying to DH that it would feel wrong spending her savings on the SDC. My grandmother would have wanted that money to go towards her own great-grandchildren. Of course she would.
I would tell my DH that we should all go away together - and OP has said the SDC can come along too if they want - but yes he/we would have to pay for the SDC outside this inheritance from my side of the family. It would be a family trip, if the older SDC want it, and they needn’t know about the finances but I couldn’t take from my grandmother to give to my SDC. They are the responsibility of their own great grandparents etc.
I don’t see what’s wrong with that? The SDC have their parents, the D.C. have their parents, each has their grandparents, it is what it is. The trouble is that there’s little that an older teen and a 6yo could enjoy together. As it happens, Disney might be one such thing (although the DH would have to tear himself in half to be everywhere with everyone, but that’s his problem for blending families, up to him to sort it out). There are other options such as a Disney cruise which could make life easier for him but that’s up to him to research and suggest to OP.