In Peppa Pig, Peppa plays a doctor and listens to the heart of his brother George, and then says "Your heart's a bit loose, I'll put a plaster on it". I wish to know what the word "loose" means in this context.
I am not familiar with this show, Edwin - I did a quick web search, so I have some idea about the characters, but only a vague one. Based on what little I know, I'd guess that since Peppa is supposed to be a young pig who doesn't know much about medicine, she (she is a female, according to Wikipedia) isn't talking about anything she understands, so she's just making up stuff. Hearts don't actually get "loose," and even if they did, a plaster wouldn't help, and we grown-ups know that. But Peppa doesn't. Perhaps she's heard someone talk about a screw getting loose, and she knows that grownups sometimes put plasters on little bumps and scrapes, so she decides to say that about a heart, too - for all she knows (she is just a little kid, really), hearts actually do get loose, and sticking plasters might help.
So I think "loose" means exactly what it always means.
A child who is not yet old enough to go to school is giving her idea of a medical diagnosis. You can assume any of the regular meanings, perhaps "free or released from being fastened or attached" (the first entry from the dictionary at the top of the page). She'll use the plaster (bandage or band-aid in American English) to re-attach it.