There are many options for a child’s theme garden. If Junior loves pizza, he could grow tomatoes, onion, garlic, peppers, and oregano. He might plant forget-me-nots at a special spot in the yard in memory of a deceased pet, perhaps with a concrete memorial he made himself. A child who adores cats might manage pussy-toes in a larger garden. She could be in charge of finding homes for extra pussies as the groundcover spreads. A small arrangement of aromatic or tactually pleasing herbs grown in the spokes of a wheel might inspire a young gardener.
How about a "farm"?: hens and chicks, cock’s comb, lambs’ ear, pussy-toes, goatsbeard, and cowpeas planted with the corn.
A "zoo" is more challenging to get together: zebra grass, tiger lilies, lion’s tail, elephant ear hosta, ostrich fern or ostrich plume astilbe, leopard plant. Enclose this garden with short stakes to suggest a cage; make them big enough and bury them deep enough for kids to walk on.
My personal favorite is a dragonland theme: snapdragons, dragon’s tongue sedum, (Powis Castle) wormwood (toxic), green dragon (related to jack-in-the-pulpit), dragon’s head, false dragonshead (also called obedient plant, and komodo dragon hosta; include a dragon statue.
To attract butterflies: butterfly bush, joe-pye-weed, yarrow, and swamp milkweed. For hummingbirds: cardinal flower, coral bells, foxglove (toxic!), trumpet creeper. For both butterflies and hummingbirds: American columbine, bergamot, hollyhock, purple cornflower. This list is far from complete; many of these plants are invasive and some are toxic, so choose carefully.
If your children aren’t interested in actual gardening, set up a bean pole teepee in a sunny location. Grow beans, or cucumbers or even watermelons on the teepee. Surround it with cherry tomatoes, carrots, ground cherries and other produce they can pick and eat while they play. This could be a separate play area, or you could center it in your vegetable garden.
Any one of these options is so much more interesting than watching grass grow---just so you can mow it.