Nearly one in five hospitals are not providing around-the-clock for children, emergency life-support cover, a watchdog says.
Inspectors looked at more than 150 NHS trusts
The Healthcare Commission also found a number of other "serious concerns" in a review of 157 NHS trusts.
The watchdog said one in 10 surgeons was not operating enough to maintain their skills and 75% of trusts were classed as fair or weak overall.
The government said improvements had been made, but more was still needed.
The watchdog has been working with the bottom 10% of trusts, which include the eight trusts ranked as weak - Brighton and Sussex, County Durham and Darlington, Kettering General, Mid Staffordshire General, North Devon Healthcare, Salisbury Healthcare, Scarborough and North East Yorkshire and Stockport.
The study into children's hospitals and those with specific children's services found surgeons in 8% of trusts did not operate on enough children to keep their skill levels up.
And just 16% of paediatric inpatient units did not carry out enough work to reach the minimum recommended professional level.
Inspectors also found only 58% of services had adequate child protection standards in place, while only a quarter of nurses and less than 10% of surgeons and anaesthetists had the right training in how to communicate with children.
However, most services were found to be making progress towards creating a child-friendly environment, including play areas.
And 99% of children requiring inpatient care were admitted into child-only wards, although for A&E and outpatients this dropped to 38% and 46% respectively.
The Commission said its report was a "wake-up" call for those hospital trusts that did not put the health of children first and pledged to put pressure on the worst hospitals to improve.
The watchdog added the findings provided evidence for the reconfiguration of services, a controversial topic in the NHS.
The report said if networks of specialist care were created it would help improve round-the-clock cover and keep the skills of specialists up-to-date.
It comes after the government set up the National Service Framework for Children and Young People in 2003 which established the standards of care the NHS should be reaching.
Commission chairman Sir Ian Kennedy said: "Children have distinct needs, they are not merely little adults.
"There remain a number of areas of variable and poor performance across the country that need to be addressed urgently."
Dr Patricia Hamilton, president of the Royal College of Paediatrics and Child Health, said: "It starkly demonstrates that NHS hospitals have made poor progress in meeting the needs of children and young people.
"This is unacceptable, but not surprising, as children's services have long been under-resourced and have not been given the priority they deserve."
Bernard Ribeiro, president of the Royal College of Surgeons said: "I want to ensure that surgeons receive the best possible training so that they can provide the highest standards of patient care and safety.
"It is vital that we surgeons receive specialist training and on the job support to carry out our roles effectively."
Professor Sir Albert Aynsley-Green, the Children's Commissioner for England, said hospitals must act on the findings.
"It is disappointing that three years after the introduction of the comprehensive standards, many hospitals are still failing to offer a safe, child-friendly service across all departments."
Health Minister Ivan Lewis said improvements had been made.
But he added: "There are still many areas where the NHS needs to improve the care it offers to children and families."
Liberal Democrat shadow health secretary Norman Lamb said the report was "disappointing".
"Crippling deficits around the country mean that hospitals are not providing the right level of all round care for children.
"Play and stimulation is vitally important in child development and speeds up the healing process immeasurably."