Matt Lucas, Harry Potter actor Tom Felton, Tom Fletcher of McFly and internet sensation Marcus Butler are among the celebrity backers. And the founders would like One Direction's Harry Styles to join the social media campaign.
Great Ormond Street will benefit from the social media campaign
"We have done everything we can to get him on board, but haven't heard anything yet," said Ms Flitterman, who is reading Spanish at Bristol University.
"We had no expectations that this would go anywhere when we started, but we have seen the ripple effect. It has not been limited to the Jewish community.
"We have seen friends and friends of friends taking part in Glasgow, Exeter, Sheffield - people Tamara and I have no connection to."
Ms Flitterman was motivated by the story of a close family member who "spent a lot of time at Great Ormond Street Hospital.
"They would not be alive if it was not for the doctors and nurses there. It was good to get the chance to give back."
Her 15-year-old sister, Izzy, has promoted the campaign at JFS, where she is a pupil. Teachers have also taken part, bringing pictures of themselves as youngsters into school. "She's also got it going at Yavneh and JCoSS," added Ms Flitterman.
Ms Fisch, a former Hasmonean girl who is reading French and history of art at Birmingham, has been heartened by the overseas interest. For example, Americans she attended seminary with have taken part.
In addition, "people who made aliyah are also looking to get involved because they all know Gosh and the work it does.
"Most people we know have a personal connection with Gosh. We all know someone who has been treated there."
She added that supporters were "uploading pictures of themselves as children with ice cream on their face, or as awkward 11-year-olds. It's really exciting - and more than that, it has given us a way to procrastinate before exams".
Ms Fisch credits her mum with coming up with the idea. "We were sitting around looking at baby photos, laughing at how different we looked and at how drastic the change was.
"My mum just said, 'We have to do something with this.' So we did. We contacted Gosh, who thought it was a great idea. We then set up a Just Giving page."
The girls became close after they were introduced by mutual friends last year - "just typical Jewish geography", Ms Fisch laughed.
Kelly Walton, fundraising manager at the hospital's children's charity, expressed gratitude "to Tamara and Emily for their fantastic fundraising campaign.
"The money raised will make a real difference to patients and their families from across the UK, helping us to rebuild and refurbish Great Ormond Street Hospital, provide vital and up-to-date equipment and fund research into better treatments for the children."
