When building circuits they can end up looking a bit like a plate of spaghetti. Un-insulated connections may cause short circuits which at best prevent the circuit from working and at worst may damage the components or the micro:bit.
Purchase insulated crocodile clips. From my experience, children are not sufficiently careful when using crocodile clips to prevent them touching each other.
To save money I recommend buying cheap, generic components from suppliers like Maplins, Radio Spares and Farnell.