In 'The Wizard of Oz' were any of the cast Munchkins actually children, rather than midgets?

No - they really were tiny people.

https://web.archive.org/web/20030206230716/http://www.chiprowe.com/bookrev/oz.html

Actually, children were used in the movie. I met one of them at an event a few years ago (Donna Stewart Hardway). She was normal height and has since passed away.

Yes, at least a few of them were children. Here's a list of ones who are still living:

http://www.imdb.com/list/ls061321507/

From there, there is Joan Kenmore who was born in 1931 and the movie came out in 1939. Similarly, there's Joan Kenmore, who appeared in the film as a munchkin when she was 7.

http://patch.com/california/lagunaniguel-danapoint/local-wizard-of-oz-child-actress-remembers-munchkin-k86c11bad86

Betty Ann Bruno (b 1930) and Valerie Lee (b 1931) also have "munchkin (child)" to their credit, so at very minimum that's four.