I'd probably just skip the party and mail a Birthday card with some cash or a gift card inside.
An adult party for a child's Birthday seems awkward.
"Come to our BBQ that also happens to be our Timmy's 5th Birthday, but don't bring him a gift because he has too much already" would feel off to me. I have a hard time believing Timmy feels the same. You only get so many kid parties in a lifetime.
That said if the party
is the gift, like a child and a few of his friends going to race go-carts or to a solarium and 3-D movie, and the adults there are more supervisory, that sounds fun. I'd probably still hand the kids Dad an envelope for my kids' share of the costs under the table.
