Many intereesting coments with very good advice.
My thoughts are more on the simple end:
I don't bring food into the 3rd level observatory or in the 2nd level control room (primarily water of coffee) - no reason the give insects or rodents a reason.
You refer to grandkids, so I'm assuming various heights. I'll keep a small 3-step ladder (with wide steps and a higher bar to hold onto) so that smaller people can look through an eyepiece.
Speaking of eyepieces, you may want to consider less expensive 2" as kids will touch these (hands, cheeks, foreheads, etc).
Your observatory size will allow multiple people at a time, so you may want to consider a few folding chairs for those kids waiting their turn to look.
I keep an older house vacuum on the 2nd level for cleaning the observatory, steps between floors 2 & 3 and the control room.
Finally, most of these items are stored outside of the observatory as I'm the only one using the room >90% of the time.
We all love to share our hobby and it's always nice to make it fun & educational, so older books and charts are good to keep nearby as well.
Stay safe and clear skies.