Depends on the family. For instance my oldest is too young to do single and my youngest is too small to ride. So in that case what would you do? Luckily my youngest is still too young to be scared, but what about in a year? My oldest will still be too young to do single rider and my youngest will then be too scared to ride Kong.
Also, the communications is the key. What are they saying to families entering. It is one thing to say the attraction is scary (most would think the actual ride and know they can do child swap) or are they warning about the line itself. Also, it isn't even about parents not riding, what about families with a mix range of kids. Let's say you have 10, 8, 6, and 4. What do you do? The 6 can't do single rider, the 4 would be too scared, and the 8 & 10 are going to want to ride. I see many scenarios where kids are crying in a line and it really isn't the parent's fault.