Completely agree with NannyOgg.
There is a perfect solution for parents who cannot under any circumstances get childcare, and that's to invite them to the dress rehearsal. There doesn't have to be any danger of any parent missing their child's performance completely.
You say they are not the human decency police, in that they shouldn't need to ask parents to leave a performance, yet you are all for them being the human decency police by preventing younger siblings at the performance.
There is a big difference between schools providing all parents with information about the arrangements for the performance they are putting on, and having to go up to a clearly uncooperative parent in the middle of a school performance, after a disruption has already occurred, and asking them to leave.