I took a look at the blog post that accompanied this image and it confirmed that the child is simply a young Sakuya and unrelated to Remilia. This makes sense within the context of the artist's comics.
Artist said: 本命中の本命、おぜうさま&咲夜さん。 +ちっこい咲夜さん。
"My favorites among favorites, Milady and Sakuya. +Little Sakuya."
seabook said: I took a look at the blog post that accompanied this image and it confirmed that the child is simply a young Sakuya and unrelated to Remilia. This makes sense within the context of the artist's comics. "My favorites among favorites, Milady and Sakuya. +Little Sakuya."
Then what's it doing in the Let's Make Touhou Kids pool?