Ok, I know it's not the same as a flower bed but since it's your daughter that expressed interest and you have sunflowers, I thought I'd throw out a REALLY cute idea I saw in a magazine last summer. I am tempted to try it myself . . .
Basically what you do is plant the sunflowers in a large square or rectangle or even a circle. Plant the seeds fairly close together (closer than package says) and leave the space clear inside the shape you make. Because sunflowers grow so fast, there is good instant gratification for kids (we grew sunflowers last year-DS loved watching them because they changed and grew so quickly). Anyway, they get so tall that once they are taller than your child, they can go inside to the middle space for their own little hidden sunflower fort. So cute! I saw pictures last year of this in both a flower bed/dirt patch type area and one that was right in the middle of the grass! I'm tempted to do it but I think DH would say no. Think I'll ask him tonight. When I saw the pictures last year, it was too late in the season but I think my kids would LOVE that! Plus it's shady during the heat of summer because the sunflowers will create shade.
If I remember right, one thing it said was that when the sunflowers reach full height, you may have to use string or something to keep them standing tall, depending on how tall and heavy they are. Then you can harvest the seeds too at the end if you want!
Not really the same idea you are talking about I know, but it just refreshed in my memory when I read your post and that you have sunflowers and I had to throw it out there. If you google sunflower fort or sunflower tent there are tons of idea and even pictures.