What manga is this girl shrinking into a child from?
The pages shown in the question comes from the short story Heart no Kusuri in the shoujo manga collection Majo no Lesson by Natsumi Matsumoto.

The two-page spread of Heart no Kusuri one-shot:



And Chinese cover of Majo no Lesson:



Summary:


  Majo no Lesson is a collection of shoujo short stories including the title short about a girl who is always teased by the boy she likes about having long black hair like a witch. She actually is a witch and her hair holds her power, but she almost cuts it when she inadvertently seals the boy's voice away.




If you're wondering how I found the manga, I split the image in your question into two, then I did a Reverse Image Search, which led me to this page. I did an Image Search once again using the cover page. Voila!