Three-quarters of older children with diabetes are not receiving key checks to keep their condition under control, a new report has found.
Just 25.4% of 12-year-olds have all seven recommended health care checks performed, according to the 2014/15 National Paediatric Diabetes Audit of youngsters in England and Wales.
Charity Diabetes UK said that if youngsters are not supported to manage their diabetes well in early life, they are more likely to be at risk of life-threatening complications later on.
Health officials recommend all children with diabetes should be assessed to ensure they are managing their condition properly.
National Institute of Health and Care Excellence guidance states all children with diabetes should have their blood sugar levels checked and those over the age of 12 should have six other regular health care checks performed annually.
These include measures of growth, blood pressure, kidney function, cholesterol, eye screening and foot examination.
Overall, the audit - which looked at data from all 27,682 children and young people with diabetes - found that most are achieving better control of their condition.
The report, published by the Royal College of Paediatrics and Child Health (RCPCH), found the average blood glucose level (HbA1C) - a marker which measures overall diabetes control - in children with diabetes has reduced for the fifth consecutive year.
The number of children who are deemed to have "excellent diabetes control" increased from 15.8% in 2012/13 to 23.5% in 2014/15.
Dr Justin Warner, clinical lead for the audit and member of the RCPCH, said it was "heartening" to see improvements but warned more must be done to tackle variation in care.
He added: "It is extremely rewarding for doctors and patients to see positive results starting to emerge after the time they have spent trying to improve diabetes care for children.
"The rate of improvement seen in England and Wales has exceeded that seen in some other European countries.
"However, we will not rest on our laurels as the variability in outcomes seen across the two nations remains excessive and ongoing improvements are still required."
On the number of children receiving all of the recommended checks, Dr Warner, added: "Regardless of postcode, children should receive the same high level of diabetes care.
"Whilst completion rates for individual care processes such as HbA1c have improved, it is completely unacceptable that the same high standard isn't reached with the six other care processes that need to be recorded.
"In reality, more children from this audit could have all seven care processes completed, service providers simply might not be writing the information down - this could be attributed to a number of reasons including a lack of time and resources."
Bridget Turner, director of policy and care improvement for the charity Diabetes UK, added: "There are still many children and young people with diabetes who are not achieving recommended blood glucose targets or receiving all the checks they should. There remains considerable variation in the level of care provided.
"This is very worrying because if children and young people are not supported to manage their diabetes well in early life, they are more likely to be at risk of debilitating and life-threatening complications in adult life such as amputations, blindness and stroke."
A spokeswoman for NHS England said: "The improvements in glucose control for children and young people with diabetes described in this report are outstanding.
"NHS England is working closely with clinical commissioning groups to continue to help them to improve the provision of effective integrated diabetes services for their populations."