It's so easy to do, and you don't need to have a big garden with lots in it - generally the only flowers that I cultivate are daisies and dandelions! Of course it's also a great activity for when you are out and about - at the beach, in the forest or anywhere really. If you want to give your children some inspiration to get started just do an image search for garden or natural mandalas, and you'll find lots of examples to give you some ideas.
I gave the children an old plate and some chalk to draw around it to give them a basic template to work with, as well as a few shells, but they didn't need much help to get started.
It was such a fun, relaxing activity that I couldn't help but take part myself. And although Mia didn't have the same patience for it as Harry, she still made a pretty good attempt, and it sparked an interest in making mud pies as she went on to mix up the flowers and stones that she had collected into various potions.
It's a bit of a shame that it's such a transient art form and they don't last long, but at least it's easy to take a quick photo of the finished work before it blows away!