Here is the "Frosty the Snowman" (1969) TV special with the original voices before it was overdubbed

When the "Frosty the Snowman" TV special first aired on December 7, 1969, the voices of the children were performed by June Foray and Paul Frees. The following year though, the voices of Karen and two of the boys were overdubbed by, respetively, Suzanne Davidson and Greg Thomas, apparently to make the characters sound more like children. — Read the rest