The study, published by the BMJ, showed that antibiotic resistance in youngsters with urinary infections is high - and could render some antibiotics ineffective as first-line treatments.
Antimicrobial resistance is an internationally recognised threat to health.
Worldwide, children are frequent consumers of antibiotics - and such routine use has been shown to increase the probability of antibiotic resistance in adults with urinary tract infections.
But little is known about the prevalence of bacterial resistance in children.
Researchers set out to review studies investigating the prevalence of antibiotic resistance in urinary tract infections caused by Escherichia coli, a bacterium responsible for more than 80 per cent of all urinary tract infections in children.
The team from Bristol University and Imperial College London also set out to measure the association between previous exposure to antibiotics and subsequent resistance in the same child.
They reviewed the results of 58 observational studies in 26 countries involving more than 77,000 E. coli samples.
The results show a high global prevalence of resistance - to some of the most commonly prescribed antibiotics in primary care - in urinary tract infections in children caused by E coli.
Within OECD (Organisation for Economic Co-operation and Development) countries, half of all samples were resistant to ampicillin (amoxicillin), a third to co-trimoxazole, and a quarter to trimethoprim. Resistance was substantially greater in non-OECD countries.
Lead author Ashley Bryce, a PhD student at the Centre for Academic Primary Care at Bristol University, said: "Prevalence of resistance to commonly prescribed antibiotics in primary care in children with urinary tract infections caused by E coli is high, particularly in countries outside the OECD, where one possible explanation is the availability of antibiotics over the counter."
Study co-leader Dr Ceire Costelloe, of the Health Protection Research Unit in Healthcare Associated Infections and Antimicrobial Resistance at Imperial College London, said: "The results also suggest previous antibiotic use increased the subsequent risk of E coli resistance to that particular antibiotic - for up to six months after treatment."
Professor Grant Russell, of Monash University in Australia, in an accompanying editorial, described how the review joins a host of recent studies, reports, and calls to action on this issue presenting "compelling evidence of the need to reconsider current approaches to community based management of paediatric urinary tract infection."
But he added: "While I have no doubt that clinical practice guidelines will quickly be able to accommodate the findings, I am less confident that there is the will and commitment to deal with what the WHO has called 'the post-antibiotic era.'"
Professor Colin Garner, Chief Executive of Antibiotic Research UK commented: "This latest research showing that up to 50% of children may be resistant to antibiotics is extremely worrying. E. coli, the cause of many urinary tract infections in children and in adults is one of the most common bacteria in our bodies. It has the ability to transfer antibiotic resistance genes easily to other bacteria and not just the E.coli species. I believe there may be considerable under-reporting of antibiotic resistant infections so that we don’t know the true level of these both in the community and in hospitals."
Dr David Brown, chair of Antibiotic Research UK’s Scientific and Technical Advisory Committee added: "It is becoming increasingly clear that bacterial resistance to our best antibiotics is more widespread than we thought. It is a problem not only in poorer countries, but also in the wealthy West. The young and elderly are particularly at risk. We need a concerted effort on several fronts to understand the scale of the problem and find solutions. Our team of national experts at Antibiotic Research UK is dedicated to doing this through research to find 'resistance breakers' to add to antibiotics to salvage them."