Two sisters have gathered more than 400,000 signatures in their petition against the use of plastic toys in children's meals.
Ella and Caitlin McEwan, aged nine and seven, want the McDonald's and Burger King giveaways to be made from sustainable materials in order to protect the environment.
They have suggested books or cardboard games as an alternative.
The fast food chains said they were looking at other options.
Ella said: "We started thinking about the environment when we were [studying it] at school.
"I like to eat at McDonald's and Burger King sometimes but I don't like the toys - they're really bad for the environment."
The sisters, from Southampton, previously appeared on the BBC's War on Plastic with Hugh and Anita.
During the programme they helped take a trailer load of plastic Happy Meal toys to McDonald's UK headquarters but were escorted from the property by a security guard.
Ella said they now plan to return to McDonald's and also meet Burger King to discuss the firms' future plans.
Their father Craig McEwan said he was "very proud" and had not anticipated the petition, which his daughters launched last year, would be so successful.
"I expected we'd get maybe a couple of hundred signatures from our our friends...and in the first week or so we did get about 1,000 but then it suddenly went viral and started shooting up."
McDonald's started giving away toys when it launched its Happy Meal deal in 1979, while Burger King followed suit in 1990 with its King Junior boxes.
Burger King said: "We are trialling the removal of toys and working on the development of alternatives, our goal being to have a more sustainable toy solution in place by 2020.
"Meanwhile, we'd like to applaud Ella and Caitlin for their initiative."
McDonald's said: "In the coming months in the UK, customers will see more books, board games and soft toys in our Happy Meals - which will see a near 60% reduction in the number of hard plastic toys given away in comparison to the first half of the year.
"This is something we have been planning for a while, and is not as a result of the petition, but we hope this reassures [Ella and Caitlin] we are working hard to reduce our impact on the environment."