A fun article on the "It's The Great Pumpkin Charlie Brown":
http://www.washingtonpost.com/blogs/comic-riffs/post/its-the-great-pumpkin-charlie-brown-7-things-you-dont-know-about-thursdays-peanuts-special/2013/10/28/abb92e4c-4023-11e3-9c8b-e8deeb3c755b_blog.htm Scroll down to bullet point number 6. They had to record some of the tougher words said by the kid actors syllabically and then splice the syllables together.