I love making 'tray gardens' with the little ones.
You use an old baking tray, biscuit lid, whatever as the garden base ...... never deep as this does imply mess !!!
Then they make their gardens, a bit of blue paper for pond or river, pebbles/ small rocks, small quantity of soil or compost, flower heads, grass cuttings and so on. Often mine add their own spiders, beetles, bugs and even the odd dinosaur!!!!!!! wonderful for making up stories.
At the same time, you could plant some fast growing plants like 'cress' on a plate for the window cill, as they do not do long term plans at an early age.
It is not designed to be kept or grown from, but after this they can progress to
'henbane's ' ideas. As it is temporary it is ideal for Nana's house.