Sunflowers! They now come in various colors & styles. They are excellent for children - the seeds are large enough for them to handle, they grow quickly with minimal care & their patience & diligence in rewarded with a big showy flower & they can eat the seeds.

think about broad beans, mangetout, French beans, runner beans as they are also good large seeds that will also show the children when some of their food comes from. Can you get nasturtium seed? or morning glory [Ipomea] seeds. Any large seed that little fingers can handle will be good.

I was going to suggest nasturtiums too, easy to grow, reasonably drought resistant, lovely bright colours and edible. (peppery and not particularly tasty but harmless...)

And agree sunflowers if the child has somewhere to plant them outside. There are some very very tall ones nowadays great fun!

Last year with my great grandchildren, pumpkin, sunflower and tomato. Small kids want something they can almost see grow. Started early luckily and then had to view remotely and pumpkins were ready by September. There are all kinds of different shaped gourds as well. I've got Turks Turban for them this year. Keep us posted what you decide on.

Welcome to GoY, Kitkat!
I would think twice about morning glory seeds. They are mildly poisonous, and in parts of Australia will be very invasive.
Since it's the beginning of autumn in Australia, I would concentrate on the cool growers: 'Sugar Snap' peas, Nasturtiums, broad beans, etc. If there is no actual frost, sunflowers and marigolds (Tagetes) can grow well enough in the winter, too.

I'd totally forgotten that you would be going into Autumn/winter. That's a good point about invasiveness.
why not ask at a local garden centre what they can suggest.