I agree this is pretty silly. It's not like getting rid of the toys will get parents to quit buying their kids McDonald's anyway, they do that because it's fast and cheap.
Which is the main issue with the notion that this will actually do anything. They can include as many toys as they want, five-year olds are still only eating McDonald's because their parents are taking them there. Obviously the kids won't harass their parents to go there quite as much now, but let's not pretend that's a meaningful factor in the consumption of fast food by youth.
I think it will have more of an impact than you suggest, really. Toys are at times a pretty big factor in kids wanting to go to McDonald's. I know I wanted McDonald's toys all the time when I was little, when they had a toy worth getting. Even if it has only a minuscule impact, it's still better than not doing it at all.
Ignoring that the responsibility for the children eating there rests solely with the parents?
It does, but parents are clearly irresponsible and stupid, anyway. (How dare I.) Removing one element of a child's pestering to go to McDonald's makes the parents, even if only very slightly, somewhat less likely to give in and go there.
If you feel that way, why not just remove all of these steps and just take their kids away? Uncle Sam can burp a baby with the best of them.....