A number of chains are still failing to serve fresh food or healthy choices, with one pudding at a Hungry Horse restaurant found to contain more than 400% of a child's recommended daily sugar allowance, the Soil Association's Out To Lunch league table shows.
The most calorific pudding on the league table of the UK's 25 most popular chain restaurants is Harvester's Chocolate Cookie Pizza, a cookie topped with chocolate sauce, marshmallows, strawberries, butterscotch sauce and chocolate buttons containing 721kcal - almost 50% of a seven-year-old's daily requirement.
Secret diners also reported that free refills of sugary drinks continue to be available to children at Frankie & Benny's and Nando's, and some chains including Carluccio's and Zizzi charge extra for a portion of vegetables with some children's main meals.
Children's meals were found to include additives linked to hyperactivity and the flavour-enhancer MSG (monosodium glutamate).
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The Soil Association found several chains had significantly improved their children's menu in the two years since the last survey.
But there remained "continuing widespread poor practice" with a number of chains failing to serve fresh food or healthy choices.
It also said chains could do more to support British farmers, finding restaurants serving potatoes grown in Egypt, apples grown in Canada, and a side salad containing ingredients sourced from 32 countries including Madagascar, Russia, Malaysia, Argentina, Bulgaria, Nigeria, Turkey, India and Peru.
Jamie's Italian topped the table for children's meals, Wetherspoons and Beefeater scored in the top five, while Prezzo and Nando's fell into the bottom five and Burger King came last.
Rob Percival from the Soil Association said: "Many restaurants are now prioritising child health and investing in healthier and more creative meal options.
"But there is still a national scandal unfolding in plain sight. Three quarters of UK parents say they are worried by the portion size of children's puddings when they eat out.
"We found that renegade chains are ignoring parent concerns by dishing up super-sized calorific junk, undermining national efforts to tackle childhood obesity."
This year Out To Lunch and TV chef and campaigner Hugh Fearnley-Whittingstall challenged chains to take action to offer puddings and drinks in a healthier portion size and to include calorie information on the menu.
In response, Pizza Hut and TGI Fridays have committed to discontinuing free refills of sugary drinks throughout their restaurants by March 2018 and include calorie information on the children's menu.
Harvester, Cafe Rouge and TGI Fridays have committed to offering puddings in a healthier portion size by March 2018.
The campaign is calling on all high street restaurants, pubs and cafes to serve two portions of vegetables with every child's meal, ensure children's puddings are an appropriate portion size and make water freely available and stop promoting sugary drinks to children.
Dr Alison Tedstone, chief nutritionist at Public Health England, said: "The restaurants taking steps should be commended but with more and more calories being eaten out of the home, big business can't rest on its laurels.
"There are still too many calorie-laden desserts and over-sized main meals targeted towards children.
"We need to see bigger and bolder commitments from restaurants and food-on-the-go chains in making their family foods healthier."
A Hungry Horse spokeswoman said: "The majority of our children's desserts are below Public Health England's guideline target and we are currently working to reformulate the range of desserts to reduce the amount of sugar they contain."
A Nando's spokesman said: "Our Nandinos menu is aimed at the under 10s, and we provide a range of different meat, veggie and dessert options, including more healthy choices such as chicken breast fillet strips, veggie strips, corn on the cob, little tomatoes and fruit ice lollies."
He added that the chain offers free water machines for customers to refill as much as they like.