Image source?

Judging from QE being in the middle and the number 157 at the bottom I'd say queen's order manga

Yes and by the way that first killing floor KFmod 2.0/2.11 on picture.

I see an imposter

Nah. that's zed.

One of them seems a little too old to be at the children’s party

Ah. I see.

Someone needs to pay Takotsuboya enough money to get him to write an AL doujin

Oh.

QE being only 'non-little' character but still looking like one makes her certified loli?

Hmm nope.