So it looks like it's about 2 out of 100, or 2% of the customer base, so to speak. To me, that is not enough to justify planning specific family-friendly events. Inevitably, if you plan family friendly events specifically to attract or accommodate these few, those will be the very events they cannot attend for whatever reason.
In this case, I think that the best policy is to have all events adults only unless otherwise specified
. And you can specify those events as they happen if they happen to lend themselves to that dynamic.
So for example, you plan the alcohol-free apple-picking day (from jmarvellous's post) and you announce, children are welcome. You plan the bike tour with dinner afterwards (from the OP) and you announce, children are welcome on the bike tour only, dinner is adults only.
And when someone asks to bring a child to an event which you have not designated as "children welcome" you just state your policy. "I'm sorry but all our events are adults-only unless otherwise noted. This event is adults only."
Having children entails sacrifice. Sometimes you have to pay the cost of a babysitter. Sometimes you miss out on stuff. That's the way it goes. And that's true for everyone, not just grad students.