Fifteen-year-old Haitham, a Syrian refugee in the Lebanese capital, starts his day at 7:00 am, but instead of carting school books, he is stocking the shelves of a supermarket.

He is one of thousands of Syrian children who have fled the country's conflict and now find themselves working to make ends meet.

Young boys shine shoes for a few dollars, while little girls beg from passers-by and sell chewing gum or roses.

There are no firm figures on the number of Syrian children working in Lebanon, but many tell stories of exploitation and abuse.

Haitham, a confident boy who looks younger than his age, works around 10 hours a day but has no fixed salary. He survives off the tips that shoppers give him to carry their bags.

In earshot of his boss, he describes his working life as happy. But away from him, the complaints spill out.

"Working here is terrible, we are humiliated and insulted," he said.

"The manager hits us, the other workers hit us... but what can we do? We have to accept it so we can continue working."

Haitham arrived in Lebanon three months ago, leaving behind a family desperate for money in Syria's northeastern Hasakeh province.

He lives with nine cousins in a single room in a partly abandoned building that is crammed with other refugee families.

His story is familiar to Abir Abi Khalil, a child protection officer with UNICEF.

More than half of the Syrian refugees registered in Lebanon are children -- some 350,000 people.

There are an estimated 150,000 additional unregistered children who are refugees in Lebanon, all of them even more vulnerable than their parents.

"In terms of impact on their psychological well-being, and social well-being, the effect is considerable," she said. "They can be severely, severely traumatised."

Along with partner organisations, UNICEF has commissioned a study to work out how many child refugees are working in Lebanon and why, as well as how best to help them.

"Whatever the reason is, if you have children under a certain age, they should not be involved in child labour... and being in the streets is one of the worst forms of child labour," she said.

Children working on the streets face physical and sexual abuse, and many were too afraid to discuss their experiences with journalists.

One of UNICEF's local partners, Mouvement Social, offers daily classes to Syrian refugee children, including those who are working.

At their centre in Beirut's impoverished Burj Hammud neighbourhood, children learn "life skills" such as hygiene but also take remedial lessons that could pave their way back into the school system.

The simple building is adorned with drawings including flowers with each petal containing a children's right.

"I have the right to learn," reads one. "I have the right to play," reads another.

Inside one classroom, a teacher calls children up to the board to practise their English handwriting.

"I miss Syria," one girl writes.

The centre caters to around 600 children a week, both Lebanese and Syrian refugees.

Rim, a pretty 15-year-old in a layered black and green headscarf, comes from Syria's northern Aleppo province.

She spent three months working at Beirut clothes shops, dealing with employers who refused to pay her wages or sexually harassed her.

In one job, "the boss accused me of being a thief, saying I had stolen and he wouldn't pay me.

"The owner of another shop would come to me and say 'I want to marry you'," she said, trembling as she described the experience.

She no longer works, coming to the centre each day instead, where teachers tread a fine line when trying to convince families that their children should be learning, not working.

"We listen to the family and listen to their needs. We want to find a solution together," Mouvement Social director Feyrouz Salameh said.

They explain the dangers that children might face in the workplace and on the streets, though in some cases the children already know all too well.

"We asked the children about the dangers of the street and they surprised us with their answer, saying 'do you mean the street at night, or the street during the day?'" she said.

"The children are aware of what happens in the streets."

Haitham, like many other children working in Lebanon, says he misses school and dreams of going back.

"I wish I could go to school, but to go to school you need to be free... so I have to forget about school for now," he said.

He isn't sure what he wants to be when he grows up, but he does have a goal.

"My dream is that one day people will be working for me, that I won't be working for them, that I will be better than them, not being told all the time what to do," he said.

"I want to be my own master."