You could add a floor plan as jpegs to your images on VRBO. If you already have a floor plan, just scan it and convert it to jpeg. VRBO's image size looks to be big enough to handle that. I used Photoshop and Dreamweaver to make a floor plan with hot spots that I put on our personal site. It's interactive such that when you click on a room or area, you will be taken to a page with pictures of that area. You could also just do a hand drawn floor plan and scan it as a jpeg. VRBO does allow you to have 2 links on your listing, so you could actually make a whole page for the plan if you have some place to host it. Here's what mine looks like: ChatterboxToo Interactive Floor Plan .
I used Paintshop pro to make floorplans of each floor, but any drawing program will work just fine -- do you have any drawing programs on your computer for your kids to use? You can also use PowerPoint (insert shapes) and then save the picture as .jpg. I display them on my own webpage and find that guests really appreciate them. Here's an example.
I do love the idea of making the floorplans clickable.