We have tried to enforce all of Hmmmmm's suggestions and they didn't work. 8 year old nephew ran, I gently restrained him and explained about no running and he didn't acknowledge me. SIL said "here, I'll get him" and then no follow-up. Nothing happened. Like I said, they don't get it. All 3 of us were uncomfortable after this.
Eden, the point of the balcony anecdote is lots of times there are some opinions, some are listened to and others are ignored. DH and I tried to establish the "buddy rule" on the balcony as a way to appease my MIL (the guest of honor) and it was disregarded. If SIL OKs a behavior, that's up to her, but she doesn't make a judgement call for every little thing they may or may not get into. Just as often there are no opinions and kids go running, and nobody says anything or chases them down.
We tried the brunch, it didn't work, it's time to move on. We will just host at a restaurant and work something out with the server that food is on us, but drinkers can start their own tab to keep our costs down. Thank you guys for the help!