If it's going to live indoors and the children are young then a curtain instead of a door and just open windows, children don't care about plexiglass.
If its being built for out door use then apply the same methods codes that you would apply to a garden shed, oh and don't forget to put some kind of hook up high to hold the door open, my daughters Wendy house doesn't have glass or plexiglass in the windows, just shutters that can be closed in the winter.