The great thing about urban farming is that you don't need things like a penflex metal hose or a huge bin of compost. You don't even need that big an area nor special, tricky equipment. All you need is some space to hang or put a pot of plant in.
Now, since summer is fast approaching, a great idea would be to make farmers out of your young ones. Choose hardy plants that don't really require much care for them to 'nurture'. Even herbs/medicinal plants like oregano can withstand a lot of pulling, digging and watering.
And... you can make things more fun by getting your kiddo a special watering can. We just bought P88 ones from Saisen, they looked like lamps and double as bath toys.
You can also use it as learning opportunities. For homeschoolers, kids can better compare if onions planted from a bulb or a seed grow better and which ones really require seeds and which require cuttings.
It can't help but be fun for them, even if it amounts to some mess and a lot of patience from you!