The word "boo-boo" was introduced to most native speakers as a child. Because of this, "boo-boo" is considered a childish term. The first definition of "boo-boo" is a "sore", a cut, or a minor injury. If you fall and scrape your knee, you have a "boo-boo." Later in life, some people use "boo-boo" to describe mistakes. (I guess you could consider it slang, childish slang.) For example: "Uh oh. Looks like somebody made a boo-boo!" or "I made a boo-boo." In the first sentence, the speaker is being a little condescending. He/She is speaking to someone who is no longer a child as if they were a child. It can be insulting if the recipient doesn't know the speaker well. In the second sentence, "boo-boo" is used to play down a mistake. "I made a boo-boo" = I made a mistake but it's not a major mistake. "Boo-boo" can also be used sarcastically in the same way. It all depends on the speaker's tone-of-voice. Also, If somebody says, "Ok. I made a "boo-boo", alright?!" They're saying, "Ok. I made a mistake! Will you please let it go?" They're are many ways to use "boo-boo", but the two basic definitions are: "a mistake" or "a cut, scrape, sore, etc."

In my part of the country a "boo boo" (rhymes with "'you") is a small mistake. A "bo bo"(rhymes with "blow'') is a child's term to refer to a small injury.

In England 'boo-boo' is used to describe a minor error. A silly mistake. Often used by adults without meaning to be condecending. It is not considered a childish term here. I have never heard it used to describe a cut, sore, or minor injury.