Here's why:
* Adjectives describe nouns: "Childish" describes a quality or characteristic of someone or something (e.g., "childish behavior").
* Abstract nouns represent concepts or ideas: Examples of abstract nouns include love, happiness, justice, and freedom. These are not tangible things that can be touched or seen.
To make "childish" into an abstract noun, you could use the noun form "childishness". This would refer to the quality of being childish.