I respectfully beg to differ. If they are alert enough to remaind you that you forgot to put your shoes in the bin, then they sure as heck should be expected to be alert enough to know when a child is about to pass through.
I've never had to put my shoes in the bin. And in the airports I fly through, the screeners stand on the gate side of the metal detector. So not only aren't they close to the belt, they can't even see it.
They way babies are wrapped going through airports, it IS possible they just thought it was a bundle of blanket in the carrier. One month babes are wee things. The grandmother was daft to say the least. I'd cut the checker some slack.