There have also been reports of some forcing teachers to recruit their pupils to be trained to fight.
The United Nations Children's Fund, Unicef said that 'a significant number' of children were killed or hurt in bouts of fighting last week at the east African country's border with Kenya.
While the exact number of child soldiers in the country isn't clear, the UN estimates there are between 2,000 and 3,000 children in different armed groups.
"We have noticed a major increase in the recruitment of children since January 2011,” a UN official told the UN news service, IRIN, adding that it came at the same time as fighting in Mogadishu and parts of south and central Somalia intensifies.
"Putting children in the line of fire, killing and maiming them in an armed conflict are among the most serious violations of international law which all parties to the conflict are expected to uphold,” said Unicef's Rozanne Chorlton.
Forty year-old mum of four, Hawa's 13 year-old son was forced to join Al-Shabab. "I have been looking for him for the past 15 days," she told the news service. "I finally got a call from him on Tuesday night and I could hear gunfire. He was in a fight. He is too young to be there. I want them to release my boy."
Many more women are finding themselves in a similar situation to Hawa. "Many mothers are like me. They are looking for their little ones. Mine does not even look like a 13 year-old. He is too small. How can they take him?"
The risk of children being forced to fight is so real that many families uprooted by the conflict are sending their children to refugee camps in Kenya or to safe parts of Somalia. "The parents cannot protect them," a local journalist told IRIN. "Any parent who tries risks losing his or her life.”
The United Nations said in a 2010 report that the recruitment of children had become more systematic and widespread. It lists the Somali government as one of 'the most persistent violators' in the world of using child soldiers, and there is evidence of several child soldiers, some as young as 12, toting assault rifles and working for the Somali transitional government in the capital, Mogadishu.