Stephany Cruz’s son, Jeremiah, who just turned 8, was thrilled by the face painting, games and pizza at his recent birthday party. It was especially thrilling because Ms. Cruz and her son were homeless at the time and living at a Win homeless shelter in Manhattan.

“He’s been counting down the days for this,” said Ms. Cruz, 27. “He was like, ‘Look at my invitation! It has my name on it!’ He was so excited.”

Jeremiah was not the only one blowing out candles at the party. An organization called the Birthday Party Project throws a monthly party for all the children at the shelter who have birthdays that month. The organization operates at more than 25 other shelters in cities across the country, offering party themes like monster mash or petting zoo.

When a family is homeless — there were 2.5 million children in the United States who experienced some form of homelessness in 2013 — birthdays are often forgotten. Even parents who are doing their best may not have the energy or resources for small luxuries. Several charities are stepping in to help. Some host big parties; others provide party materials and gifts. Another group, Extra-Ordinary Birthdays, volunteers to help parents host small and personalized theme parties for their children. “We help give the party they would give themselves,” said Schinnell Leake, the organization’s founder, and for many families, it’s a first opportunity to celebrate.

After a party at a shelter for battered women in Texas, one resident said her son ran to her, telling her he had “put out a fire” on his cake.

“My kids had never seen birthday candles,” said the mother of four children, aged 9, 6, 4 and 3. The partner she had left considered birthdays “frivolity.”

“Many of the children entering our care have never experienced celebrating their birthday or any type of holiday,” said Jasmin Reardon, who works at Helping Hand Home for Children in Austin, Tex., one of many shelters there that receive elaborate personal cakes from a charity called Bake a Wish.

She remembers seeing her first Bake a Wish cake, for a young resident’s 12th birthday. “I was stunned,” she said. “It was 2012, so naturally, it was Justin Bieber.” The cake was shaped like an iPod with the singer’s image printed on the screen. The birthday girl was proud, and her friends were “blown away.”

“We used to get all these requests for cakes that would say ‘baker’s choice,’” said Srimathi Rangarajan, president of Bake a Wish. “It’s hard for us to comprehend, but some of these children have never been given a choice in their life,” she said.

“Children don’t know why they’re not in the home they were in,” said Christine C. Quinn, the former New York City Council speaker who is now the chief executive of Win, which operates 10 shelters in New York. “They know something went wrong. They will often blame themselves, and they carry that trauma with them.” Anything that brings a sense of stability and normalcy into these families’ environment, she says, helps lessen the long-term impact of the time in a shelter.

“One way to help break the cycle of homelessness is to create a belief in community,” Ms. Quinn said. “Our residents have had a lot of people or institutions fail them.” Cakes and birthday parties, she says, “send a really important message that we think is fundamental.”

The children, though, are less concerned with the message than they are with the medium. One 9-year-old resident of the Helping Hand Home, remembering his “Ninjago” cake, said, “I think the people who made my cake are perfect!”