Why did Jimmy/Snowman not believe people when they told him the ducks on his boots were only like pictures and "had no feelings"?

Jimmy/Snowman did not believe the people because he was only five or six years old at the time. A young child's conception of reality differs greatly from that of an adult.
In Chapter 2, we learn that Snowman's earliest memories stem from when he was only five or six years old. At the time, Snowman was known as Jimmy. Accordingly, Jimmy owned red rubber boots with a smiling duck's face on each toe.
Snowman remembers that he had been fascinated by a bonfire then. After watching the fire for a time, Jimmy (Snowman) had walked through a pan of disinfectant in his boots. He remembers that he was warned to walk carefully and to refrain from splashing. The disinfectant was poisonous, and the adults cautioned Jimmy about splashing it on himself.
However, Snowman remembers that he was more worried about the ducks on his boots. Being a very young child, he did not believe that the poisonous disinfectant would not hurt the ducks on his boots.