That argument doesn't hold water when Ririha is not only well aware of the existence of lolicons but she actively tries to use her own (or perceived at least) sex appeal to win Mimika's favor. To not mention the idolization her classmates indulge in.
I mean, that's unrelated to the child model thing. And the idolization her classmates indulge in isn't too out of character imo.
Also, I dunno if you recall being a child but when I was one there were a lot of kids who tried to fake as tho they had more experience with things than they did or knew more than they did or were more mature than they were. I don't think Ririha has any intention of taking things to a sexual level. Her seduction is just emphasizing her cuteness after all. To her understanding a lolicon may just be an older person who is attracted to kids, without any awareness of all the gross awfulness that may entail.
Like, if I was going to say they're acting old for their age I'd probably say they're acting more like sixth graders than fourth graders but that's it really. In fact I'd probably go as far as to say Ririha acts more her age than Yuzumori?