I think that there are at least two ways that you could make a playhouse. One would be to make kind of a teepee frame with small poles, and then plant the sunflowers around it, leaving one space open for the entrance. Then, as they grow, just pull the sunflowers toward the poles, and tie them to the stakes.
The other thing that would work, is to put up a section of fencing in a circle, again leaving an opening for the entrance, and then plant the sunflowers around the fence, as a support. You could easily do this with part of a roll of fencing and a few metal posts, and make it as small or large as you want it to be . It would not have a roof this way, but the inside would be hidden by the flowers. I looked online, and here is a picture of one. They made it similar to the fence idea, but also planted vining plants in between the sunflowers, so it filled in the gaps .