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.