I want to make a practice cake for my GD to practice decorating on.  I tried wrapping my foam cake (bought at a cake decorating place) with plastic wrap (as the store suggested) and then covering that with royal icing.

It just barely worked.  The plastic wrap just would not attach as securely as I would have liked...so the royal icing was not as smooth as I wanted. 

GD had fun decorating it...but when I wanted to start over...I had to remove plastic wrap with all the royal on it (as well as the decorations).  And so I had to do the whole thing all over again.  If I have to do that each time I might as well bake cakes and give them away...LOL!

Is there such a thing as a "practice cake"  that one can just wash off the decorations and have a "iced" cake ready for the kid to decorate??

Why not just use a cake pan turned upside down?  Your GD will have to apply a smooth layer of icing.  And then she can pipe to her hearts content. Using royal icing means that you just soak the whole thing for an hour and the sugar falls right off.

You can also just wrap your plastic dummy completely with plastic wrap.  Cakes after all are not smooth until a coating or two of icing is applied.  So this would be a good lesson dummy regardless of its surface.

We do use the upturned cake pan approach...as well as the plastic wrapped dummy with royal icing over it.  She will practice on any surface she can get away with...LOL..but she likes the dummy cake...and Nana is trying to find a solution that she likes and Nana isn't aways tearing off the plastic wrap/royal icing. 

Seems like my cake instructor from many many years ago said that they used dummy cakes with some sort of special icing you that allowed instructors to re use over and over.  I can'r remember the name of it and I haven't seen her in over a decade and no one I talked to at a cake and candy shop outside of Seattle knew what I was talking about.  I did their suggested approach...and while it works...it wasn't what I was envisioning...a washable dummy cake that didn't need to be re iced all the time.