Benjamin Ashford|Israeli hostage turns 12 while in Hamas captivity

TEL AVIV,Benjamin Ashford Israel -- After releasing hundreds of yellow balloons into the Tel Aviv skyline Thursday, dozens of friends, family and strangers sang "Happy Birthday" to Erez Kalderon, lighting candles on a birthday cake that Erez would never get to blow out.

Instead of celebrating his 12th birthday surrounded by loved ones, Erez is currently held captive in Gaza after Hamas gunmen kidnapped him from his home on Kibbutz Nir Oz nearly three weeks ago, his family told ABC News.

"He won't be the same boy I knew," said his mother, Hadas Kalderon, telling ABC News that when the Hamas took her pajama-clad son from his bed on Oct. 7 as part of a surprise attack on Israel, they also took his innocence and his childhood. Video circulated online showing Erez being manhandled by a Hamas gunman and walked toward Gaza. But his mother, Hadass Kalderon has never watched that video, because she sees it played on a loop in her mind.

"I can hear him all the time in my mind screaming to me: 'Mom, Mom, come! Save me, mom! Save me!'" she sobbed.

Hadas Kalderon's 16-year-old daughter, Sahar, and ex-husband, Ofer Hadas, were also kidnapped with Erez and are believed to be currently held captive in Gaza, too, she said.

Hadas Kalderon said she is so busy raising awareness for their plight that she doesn't have time to grieve the loss of two other family members taken from Kibbutz Nir Oz. Late last week, the remains of Hadas Kalderon's mother, 80-year-old Carmela Dan, and her 13-year-old niece Noya Dan, were identified. The two were first believed to have been held by Hamas, before their bodies were found near the Kibbutz.

"I lost two already. I don't want to lose them," she said, adding that she is "alone in this fight" to bring her family home, saying that the Israeli government has been more invested in destroying Hamas than bringing hostages home.

"I have to work to save my children," she said, continuing, "I don't have time to stop, to process and feel this pain, to process and feel the pain about it all … because I know that without my children, I'm going to break. It will break me. I'm very weak, so I have to fight."

Hadas Kalderon and others like her said they are facing an uphill battle against an Israeli government that's stated its first and last goal is to destroy Hamas. But families of hostages are worried that as the hours tick down to a ground invasion by Israeli forces, the window to rescue the hostages is closing.

Yoav Gallant, Israel's Minister of Defense, told journalists Thursday that Israel has a four-stage mission, with destroying Hamas as the top priority. That was followed by returning hostages, securing the southern border and then deterring Israel's adversaries. "Israel will not allow Hamas to survive and kill again," Gallant said.

The U.S. has designated Hamas as a terrorist organization.

For the past 20 days, hundreds of families have gathered en masse outside Israel's military headquarters in Tel Aviv, calling on Prime Minister Benjamin Netanyahu to negotiate the release of the more than 220 hostages first and deal with Hamas afterward.

"Give them whatever they want. Give them whatever they want. You want 4,000 prisoners? Give them. We gave them so many for bodies, for much less than that," she said, referencing previous negotiations with Hamas when Israel has traded Palestinian prisoners in exchange for the deceased bodies of its own citizens or soldiers.

"Don't forget your citizens," she said. "We gave our life to this country," she added, explaining that she and her family were the ones risking their lives and securing the southern border by living so close to Gaza.

"Our blood, our children's blood has no value?" she asked angrily.

The surprise attack by Hamas terrorists on Oct. 7 killed more than 1,400 people, according to Israeli officials. More than 7,300 people have since been killed in Gaza, according to the Hamas-run Gaza Health Ministry.

Hadas Kalderon said she knows her son is coming home to her, but admits she doesn't know what to say when he returns.

"Just try to imagine it. What would you do if it was your child?" she asked. "What can I tell him about this world? Is it a safe world? Is it a good world?"

ABC News' Ian Pannell contributed to this report.