Ellen and Robert Hopkins say they were devastated to find out their son Harry had a lazy eye when they brought him for his first test when he was six-and-a-half years old.
Between two and five per cent of children like Harry are diagnosed with amblyopia every year.
The UK National Screening Committee recommends vision screening for children aged four to five, but about 11% of local authorities do not offer it in schools.
Harry has been wearing a patch for the past few months, but his progress has tapered off since he reached his seventh birthday.
"We'd definitely like to have known about it earlier," mum Ellen says.
"If we'd known about it when he was three or four, or perhaps at the start of school, then he would have had longer to work on it."
Harry's dad Robert said it was a "fluke" they brought him for an eye test, after he thought he was looking past him when they were having a chat after dinner in their home in Derbyshire.
"When I was a child I was tested in school, I can't believe kids aren't tested because with conditions like this you have a limited amount of time to make an improvement or not," he added.
Craig Murray, chair of BIOS, says it is a hard condition for parents to spot early, and that it is a "postcode lottery" for families on whether they are offered screening or not.
"We are finding more of those cases of older children who are coming in and they weren't screened, and now it's too late for them to have that amblyopia treated," he said.