I have seen it go both ways, but this was pre-COVID:

3 adults staying in 1 room, w/ a Diamond and Gold as part of the group: breakfast was free (@ Int'l Hilton property, a few domestic HGI's).
3 adults staying 1 room, w/ Diamond only as primary: 2 breakfasts were free, had to pay for 3rd (@ domestic HGI, charge was like $8 so not a real big issue).

I think it can depend on how old the kids are as well.
But in the end, I believe it is up to the property.