Cosmos - the orange and yellow ones, Cosmos sulphureus, native to Mexico - are great to put with it. Asters get bushy and big, helping to hide the bare stalks of the sunflowers. Asclepias tuberosa, the orange butterfly weed is nice. Turk's cap (Malvaviscus sp.) has cute red flowers and attractive seed pods. Many forms of smaller, more dense Helianthus to pair with the big one (I assume you are planting the kind that make big seeds and one large flower?).

Wild quinine, Parthenium integrifolium has white flowers. Dwarf joe pye weed to provide late season blooms and also Mistflower (Conoclinium coelestinum), a close relative, I think.

What zone do you live in?

Curious since sunflowers are generally annuals what difference does a zone make?

The zone is likely important for what anyone would suggest as companions to the Sunflowers.

My suggestion would be some ornamental grasses of varying forms and sizes, e.g. Miscanthus, Panicum, Pennisetum.

Thank you. I had my mind on annuals with children involved.
Children like to see things immediately. Grasses would be very nice.

I have seen at several garden shows large headed sunflowers that were only about 2 feet tall vs the really big ones. If you can find the seeds you could tell your children you are planting short ones for the toads. A small decorative toad house made by your children would be nice. It could be just a upside down clay pot

Annual sunflowers are allelopathic. That doesn't mean you can't grow companions w/ them just that they do have some impact on the surrounding plants and on some more than others.

You wouldn't want to put them in a vegetable garden for instance.

I'm hoping to grow zinnias with my sunflowers. I have both 36" and 15" varieties, so I'll put the sunflowers in back, the tall zinnias next, and finally the short zinnias.

Martha

Cosmos are nice and easy to grow. As a poster suggested , they can be both a filler to hide bare stems of sunflowers, and they bloom beautifully. I like them mixed colors, white, pink,..
I would try to plant sunflower/cosmos arrangement toward east / northeast side of the garden, so they wont shade other shorter flowers/plants.

Wild Flowers gardening is an art form. There is no right or wrongs. It resemles to painting. But you have to pay attention to plants' requirements;
like, sun, shade, partial, soil type, watering, etc.Best one can do is to mimic the nature in a wild flower garden.
I believe that there shouln't be an obvious design and pattern in order to look more natural.
The reason I am suggesting these because this is wild flowers forum.