Dutch door?

My husband and I are in the process of building our first house!
We will have a playroom for our 3 year old son and want to have a dutch door for that room- the bottom half can stay closed so he and the mess stay contained by my husband and I can still see into the room.

These doors are very expensive to purchase. I can see that they would probably be fairly simple to make myself, but I have no idea where the cut would go and how I would close the top half of the door when I wanted it shut.
Does anyone have any plans for a DIY dutch door?

Thanks!