Perhaps they are wealthy, perhaps they are merely well off, like Princess Margaret’s children, or perhaps they are not so well off (like some of the Queen’s cousins) it’s hard to know as the terms of any trusts which may benefit them aren’t open to public scrutiny.
They will be beneficiaries to their father’s lease at Royal Lodge @ some point (assuming he does not remarry and leave a widow) which has 60 years to go at this point - not sure they’d want to live there together or to assume responsibility for paying for the upkeep/running of the property, tho’.
Beatrice, especially, has a rather expensive lifestyle and how much her personal assets could support that w/out funds from her father isn’t clear to me.