It never occurred to grandmother to ask her son who this little girl was???
What did grandmother know and when did she know it???
From what I read in other articles, he called the girl "his girlfriend's daughter" (referring to Amanda as his girlfriend). What mom would investigate beyond that? Ariel apparently liked kids and was good with them. No reason to be suspicious that he was entertaining the little one for his GF while she was "shopping" or whatever convenient excuse there was at the moment.
Or are we just looking for as many people as possible to blame for the next few weeks in this story? Cleveland PD is already clearly going to get a lot of Heck for not getting this one sooner.