Eleven-year-old Mohamad Firman Shah was running about at a playground last month when he accidentally stepped on a woman's foot.

She was so angry that she called the police and paramedics even though his mother explained that both she and her son were visually impaired.

"People do not know we are visually impaired because we do not look like we are handicapped," said housewife Norrizan Nahar.

The 35-year-old mother of six has congenital cataracts, a clouding of the lens of the eye that is present at birth. The condition can affect one or both eyes.

When a child has a cataract, the lens cannot focus the rays of light onto the retina normally. Instead, the lens produces blurry images.

Her daughter, Anggun Kasturi, 14, and son Firman were born with a similar eye condition, and had cataract removal surgery when they were a few months old. Both wear glasses for long-sightedness, and need regular eye checks to ensure they are wearing the correct lenses and receiving appropriate treatment for lazy eye.

"It is a rare condition that occurs in one to three in 10,000 children worldwide," said Dr Cheryl Ngo, Consultant at National University Hospital Eye Surgery Centre. Both Singapore National Eye Centre and KK Women's and Children's Hospital operate on fewer than 10 cases a year.

Except for the thick glasses that the children wear, their visual handicap is not apparent. But just to make out the information in things like text messages and on price labels at the supermarket, they have to make sure the words and numbers are extremely close to their eyes.

Madam Norrizan had a corneal transplant in July 2013 that marginally improved her vision in the right eye. But she was found to have corneal graft rejection in May 2014 which left her with exceedingly low vision, comparable to that of a newborn baby.

The former hospital porter and cleaner now relies mainly on her left eye for sight and has since been certified medically unfit for work.

Madam Norrizan, who is a single mother, currently receives social assistance that helps her with medical expenses, rental for her two-room Housing Board flat, and household utility bills. She also collects food rations every month from the community centre. Her oldest daughter, Anggun Sulastri, 16, takes care of daily household chores and her younger siblings.

Even as they get by with the bare minimum, Madam Norrizan's children are her motivation in life.

When Kasturi and Firman are occasionally teased for their thick glasses and "big eyes", she tries to encourage them to be confident because she had similar experiences growing up. "I always tell them to be positive because we still have our hands and legs. Only our eyes are handicapped."

Despite her visual impairment, she tries to take the children out at least once a month to places like West Coast Park and VivoCity mall, where the children can have fun and get out of the house.

Madam Norrizan used to be a sprinter with the Singapore Disability Sports Council. Her hope is in her children and she wants to see them "be somebody some day".

Inheriting her mother's passion for sports, Kasturi also loves running and trains in school every Wednesday. "Kasturi wants to be like me," said Madam Norrizan. "She said that she wants to join the Paralympics and represent Singapore one day."