If the definition of a noun is a person, place, thing or idea....?

Knowledge, honesty, intelligence, cloverness,bravery, goodness,kindness, wisdom..... these are called abstract nouns

No, you can't buy a creepy, you can't go to a creepy, you can't call a creepy on the phone. An Idea can be creepy, but the Idea is the noun creepy describes the noun.

Since "idea" is not a person or a place, tell the 8 year old that if you have an "idea" in your head, then you have someTHING in your head. "Idea" belongs in the THING category.

In the definition you give, "thing" seems to refer to a physical, tangible thing (e.g. kettle, goat, table). That would separate it from an "idea", by which they probably mean "abstract concept" (e.g. love, fear, communism, ethics, language, mathematics).

Love. Not as in "I love you" where it's a verb, but as an idea like "I need love in my life".

...an idea

happiness

freedom

liberty

socialism

childhood

justice