My friend gave me some Little Garden seeds from M&S and we had great fun planting them back in March. They were super easy for little hands and it kept her engaged for a good hour.
You can't get your hands on the kits anymore but I have found an alternative here.
It came with a little tablet which you wet and it expanded into soil. I've never seen anything like this, Daisy loved it, shes 3.
They were super easy to look after. I put them on my kitchen windowsill and watered when they felt dry. When they got too big I transferred them to plastic pots and now its warmer (in June) I've left the, outside.
It was great for Daisy to watch them grown and look after them. We have flowers, Radish, Pepper and some herbs.