1. Make sure you keep the alcoholic drinks far away from the kiddie table and preferably serve them in something other than sippy cups.
2. Have lots of toilet paper, wipes, and a plunger handy in the bathroom.
3. Realize that some adults behave like children and some children are mature beyond their years.
4. Hire a clown for the adults and a fortune teller for the adults. Wait, reverse that!
5. Have fun and the rest will follow.
In all seriousness, I grew up in a large extended family. No one ever made a big deal about gatherings being for both kids and grown-ups, that's just how things were. As kids we made our own fun and it was fine. Now as a grown-up with kids, I'm always surprised by how few gatherings/parties I get invited to are actually family-friendly. It's a shame. Adults should really learn to play well with kids, it's not that hard. Just my opinion.