how many costumes do the von trapp kids typically have? (my daughter will be Gretl and I am wondering what is typically done)

So far I have counted these:
-sailor suits
-nightgowns
-curtain outfits
-dresses for "farewell goodnight" party scene
-austrian outfits for the festival

Do they also need:
-separate dresses for the "sound of music" scene while singing with captain
-dresses to wear during wedding scene

You've got all of them. And yes, typically there are "bridesmaids/groomsmen" type outfits for the wedding. And cloaks for the trek out in the mountains at the end (over their festival costumes).

I really don't remember the captain/SOM scene in the show. The memory of the movie is overriding my memories of the four productions of SOM I've done. Funny, huh?

thanks so much! I found out that she will have the sailor suit on when the kids sing "sound of music with the captain". Its a scene right after the captain yells at maria for having them running around in "drapes". They start singing to the baroness. We are super excited, the show is about 5 weeks away!

I needed infant and toddler costumes. I must say  http://www.halloweencostumesale.com/  provides  a large selection of adults and kids' Halloween costumes. What I loved most is that you can order online. You can found cheap Halloween costume ideas also there.

any latest sale?? waiting