I publish in the Children's categories as well, (Since ePubIt) and the short answer is that I have never found my book by category. I did browse a very long general list and found my book very, very deep in the listings.
Searches are quirky, but your book should be available by searching specifically for it.
Every book published by a publishing house will come first. When you are Nook Press author it is all about marketing yourself heavily to get traffic to your book, and eventually sales. THEN B&N will give you more attention.
If this has changed, someone please chime in.