Here are some things that will help to get you started gardening with your young children, and some things that will make it all a lot more fun:
A decent instruction book
It can be difficult to know where to start, so look for a book with plenty of simple instructions that is particularly aimed at parents of toddlers and young children. It will give you some ideas for the types of plants that are easy to grow and will give you good results.
There's quite a size difference between adult and child hands, so it's important that young children have garden tools of the correct size to work with. It's nice to have a set which fits into a handy container that children can use to carry them about and store them in.
Gloves will protect children's hands and stop them getting quite as messy as they would otherwise!
Not just for gardening, children will love using a toy wheelbarrow to wheel their toys around the garden! It's worth getting a decent one as it will have plenty of use and you want it to be able to hold a reasonable weight.
We bought our toy lawnmower very cheaply some years ago, and it has easily become the favourite garden toy. Your children won't be any actual help to you while they are using it, but we've found that an older child will happily trot alongside an adult that is really cutting the grass and it's a great way to occupy them while you know exactly where they are!
Finally, children love these bright and cheerful pinwheels. They are a lovely decoration for the garden, and very young children will be transfixed by all those bold colours whirling round and round.