Shalabi has refused food for 43 days since Israeli forces detained her from her home in the northern West Bank on Feb. 16. Israel has sentenced her to four months but she has not been charged with any crime.
Children released balloons decorated with Shalabi's face, sang and waved Palestinian flags.
They carried banners reading: "We are all Hana Shalabi," and "The children of Palestine will not give up on you even if the whole world does."
A young girl delivered a speech calling on the children of the world to join her in demanding Shalabi's freedom. "When is the world going to act?" she asked. She said the children of Gaza supported Shalabi and all Palestinian political prisoners.
Shalabi was hospitalized on March 20. A doctor from Physicians for Human Rights - Israel visited her on Monday and said her muscle atrophy had worsened, and was affecting her heart muscle.
An Israeli military court on Sunday rejected Shalabi's appeal against her four-month sentence.
The judge claimed that a medical report did not provide information to suggest her state of health was a cause for concern, Amnesty International said.
Lawyer Jawad Bulus, a member of her legal team, said he submitted a petition to the Israeli high court on Monday demanding her release.
Amnesty called on Israeli authorities to either release Shalabi or charge her with an internationally-recognizable criminal offense and bring her to trial in accordance with international standards. Gaza children demonstrate for Shalabi's release