First, the literal meaning of a live wire is an electrical wire that is "live" i.e. there is electricity being drawn on the circuit. Do you know what happens if you touch a live wire? You get an electrical shock -- your body jerks around and you shake and tremble... That is not the meaning of this expression exactly, but bear that image in mind... A live wire is someone who is hyperactive, moves quickly, suddenly (similar to the image above) but there is ALSO the idea that a live wire is someone who has an offbeat sense of humour. Someone who is an entertainer ... makes jokes (not always funny jokes) A live wire can also be someone you don't want to get too close to ... because (as with a real electrical live wire) they can be unpredictable and dangerous... If someone calls you a live wire, that's not a good thing -- that's just my opinion. The connotation / flavour of this expression would be very context-dependent.

If someone is a "live wire" it means they are unpredictable and energetic. It applies to anyone, not just children and can be used both negatively and positively. Example: "I can't believe she did that!" - "Yes, she's a real live wire."

Children are like live wires= very active and energetic

to live wire - not to be able to stay still, to be active all the time, has to do something all the time, can not rest, always going around, always doing something