The source sentence is rather odd because ふにゃん refer to the dessert's soft texture, not the sweetness of the food. But what it wants to say is that the warabimochi is soft and sweet. ふにゃん is a derivative of ふにゃり and ふにゃり and ふにゃん are often used to describe the soft texture of warabimochi.

「指で突付いてみると ぷるん　とした弾力と　ふにゃり　とした柔らかさが感じられました。」
http://www.b-shoku.jp/modules/wordpress/?author=2177&p=87659
http://ejje.weblio.jp/content/ふにゃり