Mobile phones and other electronic devices should be banished from the bedroom at night and the dinner table, according to new guidelines on screen time and social media from senior medical advisors.
They also say children should take a break from screen-based activities every two hours.
Adults should also “lead by example” with their own screen use and online behaviour, the chief medical officers of England, Northern Ireland, Wales and Scotland said.
They also said technology companies must do more to keep children safe.
The report comes as England's Health Secretary, Matt Hancock, is set to meet bosses at Instagram over the handling of self-harm and suicide content.
Prof Dame Sally Davies, England's chief medical officer and the lead for the UK, said companies had a duty of care to help keep children safe and that age limits for using social media needed to be properly enforced and children should not be channelled towards harmful content.
But Dame Sally said a review of evidence had not proven a clear link between screen-based activities and mental health problems.
The guidelines suggest:
Not using phones and mobile devices at the dinner table - talking as a family is very important for development
Keeping screens out of the bedroom at bedtime
Talking as a family about keeping safe online and about cyber-bulling and what children should do if they are worried
Not using phones when crossing a road or doing any other activity that requires a person's full attention
Making sure children take a break from screens every two hours by getting up and being active
Policing their own use too - parents should give their children proper attention and quality family time and never assume they are happy for pictures to be shared.
The chief medical officers' guidance comes after leading paediatricians said last month that parents should worry less about screen use after a review found there was little evidence it was harmful in itself.