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. Above is the original, reconstructed with Foray and Frees's voices (and some 1980s commercials included in the recording). Below is a comparison video. From Lost Media Wiki:
Since Foray's name still appears in the end credits, Davidson never received public due until the 2018 Deluxe Edition DVD and Blu-Ray name-dropped her in Mark Evanier's audio commentary. Foray recalled her experiences in the book The Enchanted World of Rankin/Bass:
"I was called in to voice the little girl Karen. I was disappointed to learn later that my work for the Karen character was replaced by another actress. To this day I am unsure of the reason, but I still enjoy the special."
Foray's voice can be heard on the Rhino Records soundtrack.