The futures of so-called "lockdown babies" are "at risk", the NHS regulator has warned as the number of infants attending A&E soars.
Some 2.6m children under the age of four were taken to emergency departments by parents in 2022-23 - almost 50pc higher than a decade earlier.
The Care Quality Commission's (CQC) annual report into the state of health and social care across the country found that children's NHS services were struggling across the board.
It comes as more evidence emerges of the damage that Covid-19 lockdowns did to young children, who are starting school two years behind and less likely to be toilet trained, well behaved and able to communicate with teachers and other pupils.
The report suggests that without immediate action they could now face a lifetime of health issues too.
The report, which used data analysed by the Institute for Health Visiting, said 20 per cent of A&E visits by under fours were avoidable if there had been better community support, but there was a lack of community nurses and other local services.
Children under one were the most likely to attend emergency departments because of rising cases of coughs, colds, stomach bugs and difficulties in feeding, it was suggested.
In one example, in north-west London, 59 per cent of babies who attended an emergency department in 2022 did not need treatment and were sent home after reassurance. This cost an estimated £1.8 million a year in this one area of the capital alone.
Ian Dilks, chair of the CQC, said that "failing to ensure good, safe care for our children today also risks failing their future".
The report also revealed that 3,743 under 18s who died in 2022-23 was 8 per cent higher than the year before, and the highest since the children's mortality index was established in 2019 to try and establish the cause of early deaths.
It said children were more likely to experience delays in accessing health care, which were likely to cause "significant and lasting consequences".
The Royal College of Paediatricians previously raised concerns about the 400,000 children waiting for hospital appointments with a consultant in May 2023.
While this had reduced slightly to 363,000 by June this year, the CQC was concerned that "over 140,000 of these had already waited over 18 weeks".
Elsewhere, it found mental health services were a "particular concern", and unable to keep up with soaring demand with one in five children aged eight to 25 declared to have a mental health disorder.
A lot of mental health issues in adulthood stem from formative years, and so prevention is key, it said.
In its work it found that while the workforce had grown, "problems with staffing and skill mix remain".
"Across the country, services are facing challenges in recruiting staff including nurses, psychologists, occupational therapists, and consultant psychiatrists - all of which are having an impact on capacity, and therefore on the availability and regularity of appointments," it said.
Young children's inability to get a dentist appointment is also having a "clear impact on children and young people", the CQC said, with parents feeling pressure to go private.
The watchdog asked 1,000 parents and carers of children about access to health and social care services, with 31pc saying they had difficulty accessing dental care for their child.
Parents said appointment times - usually offered during the school day - are inconvenient, with instances of youngsters being left in pain due to a lack of emergency appointments.
According to the report, there were examples of "some practices appearing to steer patients towards more expensive private care".
One parent told the CQC their dental practice had said there were no appointments available for a number of months for their 15 year-old, but if they went private and paid £30 they could have an appointment that same week.
Mr Dilks said this year's report had shown a spotlight on "the many children and young people who are not getting the care they need when they need it".
"While some children are receiving timely, appropriate care, we know that there are more who don't, with potential long-term repercussions for their mental and physical health," he said.
"The health and wellbeing of a nation's children has been described as the best predictor of its future prosperity; failing to ensure good, safe care for our children today also risks failing their future."
An NHS spokesman said: "This report emphasises what staff and patients unfortunately already know too well, that right across the NHS, services are facing incredible pressure and capacity challenges with waits that are too long - particularly when it comes to the care of our children and young people.
"As well as NHS efforts to bring down waiting times for planned children's care, mental health teams are treating more young people than ever before and while we are working as fast as possible to increase capacity including the roll out of mental health teams in schools, there is much more to do.
"Improving the recognition and response to patient deterioration - including symptoms of sepsis - has also been a priority for the NHS in recent years which is why we have supported the national implementation of a Paediatric Early Warning System, alongside rolling out Martha's Rule at over 140 hospitals, which will enable patients and families to seek an urgent review if their condition deteriorates."