The research found that the average child consumes the equivalent of 11g of sugar at breakfast and by the end of the day more than three times the recommended limit.
The study, taken from the annual National Diet and Nutrition Survey, also found that the majority (eight out of ten) thought their children's breakfast was healthy.
PHE has now launched an app in a bid to combat health problems linked to sugar intake. The app enables parents to scan barcodes to see how much sugar, salt and saturated fat they contain.
Alison Tedstone, chief nutritionist at PHE, told The Guardian: "Children have far too much sugar, and a lot of it is before their first lesson of the day.
"It’s crucial for children to have a healthy breakfast, but we know the mornings in a busy household can be fraught. That’s why we’ve developed our Be Food Smart app, taking some of the pressure off parents and helping them to choose healthier food and drink options for their children.”
The recommended daily sugar intake for four-to-six year olds is five cubes of sugar, and for seven-to-10 year olds it's six cubes.