In the study, published in the journal Pediatrics, parents who were offered a genetic test supported their children also being tested.
The authors say doctors and politicians need to be more aware of the issue.
Genewatch UK said children should never be tested for adult conditions.
Genetic testing used to be confined to specialist clinics, but direct-to-consumer testing is now possible.
People send a sample to a company in the post and are told if they have any genes which carry an increased risk of illness.Genetic test
In this study, 219 parents were tested for 15 genetic variants linked to heart disease, osteoporosis, high blood pressure, high cholesterol, type 2 diabetes and colon, skin and lung cancer.They were then asked a series of questions to compare benefits such as reassurance, knowledge and prevention, with risks such as invasion of privacy and psychological discomfort.
The report found that "parents offered the genetic susceptibility test for common preventable conditions tended to consider that the potential benefits of this test for their own child could outweigh its risks".
Professor Kenneth Tercyak, from the Georgetown Lombardi Comprehensive Cancer Center, said: "These tests usually don't offer a clean bill of health and can be hard to interpret even in the best scenario."
Helen Wallace, from genetic science lobby group Genewatch UK, said: "Online gene tests frequently give misleading results because most common conditions such as cancer, obesity or diabetes are not predicable from a person's genes, except in special circumstances.
"Children should not be tested for risk of adult-onset conditions, full stop. They should be allowed to decide for themselves, with medical advice, when they are grown up."
Dr Vivienne Nathanson, director of BMA Professional Activities, said: "We would have concerns about genetic testing being widely available over the internet or off the shelf because parents could find out results without a health professional to help intepret them. They may also find out about genetic abnormalities for which there are no cures, or be caused needless worry.
"It is important that parents who find out that their children have a genetic disposition to a particular illness, have counselling in advance so they understand the consequences of the test for their child, other children and themselves."
Prof Tercyak said: "The findings of our study should remind clinicians and policy-makers to consider children when regulating genetic tests."
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