Anyone who cares this much about single riders messing up their photos (and I'm not talking about seating arrangements but the more recent posts) should have the photoshop skills to erase them. I mean, seriously, if having another person in the pictures is THAT big of a deal, just put down the money for the software and use it.
I have an acquaintance who erases EVERYONE else from her photos. I don't understand it at all. If she wants her daughter to think they roamed the earth all alone, once my friend is gone and it's just her daughter, then it makes sense, but it just seems weird and controlling to me. For me, I don't really care who else is in the photo. If it matters, I just wait until the person has moved.
And as a regular single rider line user (at DL more than WDW since WDW has so very few), they will be placed AFTER your family, so just arrange people as you need.
As for not being able to see someone, that can be part of your memories, too. Watching the child grow through the years in TT shots (if you keep buying photopass stuff...I gave that up years ago, what a waste!) could be fun. First the child was invisible and then years from then is towering over dad? Memories.
I like photos that are in context with the actual events; not a whitewashed version.