A restaurant in our neighborhood has a kid-sized door much to the delight of both diners and passersby, but we imagine having one inside your home would be a loads of fun for kids.
We wish we could tell you more about this particular door setup, but it's on a foreign website (Turkish?) that we haven't been able to translate. As parents, often we want to keep young kids contained with a door and having a door knob out of reach works in our favor, but imagine this diminutive door between siblings' rooms and all the games and fun that could entail. Check it out at Tasarim.