My first go would be cress and sunflowers. Do you have a sprouting kit ? Sprouting can be interesting too !

Carrot tops are brilliant for this. Just slice the top off carrots, place in a saucer on a wet tissue. Keep wet. You won’t get a carrot, but it grows fast. And those stockings with dirt and grasss seeds also grow fast. Also no product at the end. Maybe the carrot tops as well as seeds in the garden. That way they have something to see while waiting for the productive seeds to sprout

At school the teachers would plant grains of wheat because they grew very quickly.

Try buying established seddlings. Have plants that grow at different rates. Herbs are great. However patience and reward counts too. Your toddler will be part of this process

Woolworths have great kids garden packs at affordable prices.
Not sure on how quickly they take off but they must be pretty good because they are selling like hot cakes!
There’s a range of different packs so you have a variety to choose from or you can get one of each.

Cress is a great starter and it grows quickly so they won’t get bored waiting for results

Snow peas. They grow quick, you almost see them grow.

Kmart and big w have starter kits for little gardens you start growing inside we got the butterfly one (from kmart i think) it has cosmos and poppys in it. its excellent you dont need to do much and the cosmos especially will pop up almost over night then you plant the whole thing in a bigger pot or garden and bam it takes off great

Oh & I forgot we found some seeds at bunnings for round ball shape carrots that were meant to grow fast. I think they called them French Carrots. We had a few really hot days & our seedling didn’t survive so can’t say how they went sorry.

Cherry tomatoes, my daughter loves picking them.

Visit your local Plant nursery and ask them to help you. Im sure they could give you many options.
You could take his interest and get him growing food as well. Bean Sprouts and alfalfa grow fast.
You can get mushroom kits and grow them inside in a dark cupboard.
Sunflowers grow quickly also.

Try sun flowers. You can get little kits with everything you need from most garden stores, and even Big W and Kmart.

Thanks so much for the suggestions. I’m a novice to gardening so had no idea about quick growing plants.

What about dwarf beans? you can put seeds on a saucer on top of damp cotton wool, Keep the cotton wool slightly damp. I don’t know whether you can transplant them into the garden or not. I always planted them straight into a garden bed in a sheltered position where they got morning sun.
My niece put some bulbs in pots and the children are allowed to water them under supervision. The youngest child is 2 1/2.
Any chance of getting some cuttings of plants from somebody else’s garden as a trial. The best is a small cutting that is reasonably drought tolerant considerring water restrictions. You are more likely to have success with a small cutting than a big one. Perhaps geraniums with different coloured flowers. I don’t know what grows in your area so it is hard to know what to suggest. Maybe go for a walk in your area and see what other people have growing in their yards, either in the ground or in pots. It also depends on your soil what is suitable to grow if you want to put something straight into the ground. You could go to a garden centre in your area and get some ideas., even if you don’t buy plants then. Depending on your climate if you are growing it in a pot or hanging basket as long as it gets protectipon from the hot sun in summer, zygo cactus are ok. They don’t have any prickles at all. When they grow big enough you can break a small piece off and start another plant. I was given some cuttings about 12 months ago. I went away, it came out very hot, they didn’t get watered at all. Some survived, some didn’t. I still have some cuttings in the kitchen in a glass which I keep about an inch of water in. They have roots on them and some of the cuttings actually flowered – inside in the kitchen. It doesn’t get steam or too much heat from cooking. I am still very surprised that they have survived though, especially as it was so cold this winter I had the heat of the airconditioner on but the plant wasn’t too close to a vent. Last summer during a heatwave that lasted over a week I had the airconditoner on, at one stage for 3 days non-stop as it was up in the 40s, I feel the heat badly and was told by the doctor to keep cool. Admittedly the cuttings don’t seem to have grown a lot in height but they have put more side shoots on.

wheat grass grows pretty fast
you can also regrow store bought celery once the stalks are all chopped off by placing the end in a shallow dish of water. They shoot in 3 days!