I'm amazed at the creativity of people who build playhouses for children. When I was a child, we had an adorable log cabin in our backyard, with open windows and a doorway. It was very simple and probably not very expensive, but I remember spending hours and hours playing in it. In my opinion, it's better to build a simple playhouse because it encourages the children to use their imagination more.
In addition, since it's probably only going to be used for a few years, it's better not to spend too much money on it. Check your local classified ads for used playhouses. Even if it requires a bit of fixing up or remodeling it's less expensive to fix a few broken boards or even get a replacement window.
I found some really amazing playhouses on Pinterest that are great inspiration for creating play spaces for children in all ranges of budgets, from using everyday things found in nature to playhouses that even have electricity. Most of these ideas could be easily scaled down to a more affordable option, but it's almost as much fun to browse playhouse ideas as it is to browse online for homes for sale.
There are a few things to consider when you are creating a playhouse for a child. First, you will want it to be large enough to accommodate at least two children, preferably four. You will also want it to be tall enough so the kids don't have to bend over when they are inside, unless of course your goal is to provide a quiet place for sitting. If you choose to build with a theme of some sort, be sure to leave it open enough for the child to have lots of brain room for imagination. Think of it this way - many times when a child opens a present, the box is played with just as much as the toy!