Children designing a “no adults allowed” garden at the Chelsea Flower Show have been persuaded to let grown-ups into the exhibit – but there is a catch.

Adults who want to see the show’s first garden to be designed by children must pledge to plant a tree, donate to a school gardening campaign, or find a flower that starts with the first letter of their name.

Clare Matterson, director-general of the Royal Horticultural Society (RHS) which puts on the world-famous flower show, said she had secured access for adults to the garden after some “tough negotiating”.

The RHS No Adults Allowed Garden has been co-designed by nine- and 10-year-old pupils from Sulivan Primary School in Fulham, south-west London, and garden designer Harry Holding, and will feature at the Chelsea Flower Show in May.

The garden, which will not be judged, features a “fantastical” landscape with lush woodland, bountiful meadows and a wetland with oversized bog plants, with a final destination of a natural den set in a pool of water.

Adults will have to share what they have pledged to get a sticker designed by the children in order to enter the garden.

Originally, we only wanted children in the garden, so that we could keep it to ourselves and run around

The RHS and Mr Holding have held planning, design and creative sessions with children in the run-up to the Chelsea Flower Show, including planting up herbs, learning about balcony gardening and garden design, designing trophies, and arts and crafts.

In a statement, the youngsters from Sulivan Primary School said: “Originally, we only wanted children in the garden, so that we could keep it to ourselves and run around.

“But the class also wanted the world to see our hard work and how great a children’s garden could be.

“We thought we could use this opportunity as a chance to help the planet, convince other children to become gardeners and to have some fun.

“So we have agreed to allow adults into the garden as long as they complete one of our pledges.”

Ms Matterson said: “I’ve done some tough negotiating in my time and this was no walk in the park, but the children and the RHS are all delighted with the results.

“The Sulivan school pupils came up with the three activities, which they’d like adults to pledge to do one of, and we’ll be asking visitors what they did or plan to do before they enter the garden at the show.

“We know gardeners are a good bunch and so we trust that people will do what they pledge and it’s all in the name of having some fun too.”

The children chose the pledge to plant a tree because of their importance for the future of the planet and their value to wildlife, and the donation to the RHS campaign for school gardening so other youngsters can gain access to nature and garden activities.

And they chose the third option, for adults to find a flower that begins with the first letter of their name, because they believe learning should be fun, Ms Matterson said.