Another who can't see the issue with this.
Ideally, everyone would have said "let's all club together", but, as they haven't, then working out a sort of 'per head cost' sounds like a pretty sensible way to go.
We're fortunate in that our finances can stand inviting adult dc to Christmas, but if I were in a position where I couldn't, I'd not hesitate to say "I'd love to get together this year, and am prepared to have you all here, but we need to share the costs"