Siân has won the endorsement of Harry Bear, the first teddy bear to become an MP.
Harry, the creation of author Richard Heller, works to make politics more accessible to children.
His backing comes after Siân promised to ask London’s children to write her three new policies to make their lives better if she becomes Mayor.
Mr Bear said: "I welcome Siân’s decision to let London’s children create three items for her manifesto. This will be the first time that children have been given any say in a British election. I hope that there will be many more. In view of this excellent news, I have withdrawn my own bid to become Mayor and will be making Siân Berry my first choice for London."
Siân said: "Harry’s message is that if we want to do the right thing for our children, we have to really listen to them – I couldn’t agree more, and I’m delighted to have his support. A Green City Hall would give all Londoners a real say in the decisions that affect them, and I wouldn’t leave anyone out just because they were too young to vote. I’m delighted to show my commitment to that by leaving a space in my manifesto for three new policies from London’s children.
"We also agree on policies to give children a better school experience. I’ll fight the sell-off of playing fields, and look for derelict land to turn into new recreational space. I’ll let children give their verdict on their schools, and publish the results alongside exam scores and parents’ choices. I’ll revolutionise voluntary sector funding to give London children more opportunities to take part in sport, the arts and other activities. And of course, I’ll keep campaigning for votes at 16."