Ok, Now being British I note that most of the replies are from US based speakers of English .. BRAT for me is a negative term and I can't think of one occasion when I'd use it as a playful Positive term.

An “army brat” is the child of a career soldier, which doesn’t have a negative connotation. The more usual meanings would be at least mildly negative.

You can only use this in a playful / joking manner with people who are very close to you (close friends or boyfriend). For example: Boyfriend: "I went shopped for the past 3 weeks, can you do it this time? I have to go out for beer with my college friends tonight." Girlfriend: "You are such a brat! You always have a reason to drink with them!" Also, I see that you are from Peru! If you are interested in English-Spanish language exchanges, whether it is sharing audio clips or revising each other's writing, send me a message, and I will give you my Skype / Whatsapp. I am a native English speaker and I would like to improve my Spanish level from intermediate to advanced

You can say it playfully, depending on your intonation and your relationship to the listener. It tecnically is negative but most English speaking countries have a culture where if you say a negative word with a friendly intonation it can make it endearing, playful etc. (it changes the syntax) most negative words can have positive syntaxes in English Example: girl: "hey bitch" ☺️ other girl: "hey!" 😊 "bitch" is a deragatory term but in this context it is used endearingly. The syntax in English within most major cultures is so that intonation and relationship has meaning on the word. It is a cultural thing.

Yes, it's a mild way of saying "spoiled", but it is only used with people you are familiar with.