Black pants and hoodie
5 meters of strip LED lights in your choice of color(s). The length will ultimately depend on the size of the person wearing the costume but 5 meters is more than enough for a child.
Electrical tape (black is typical but I had both black and clear and used both)
9V battery connector and battery OR 3 Cell LiPo battery and appropriate connector (LiPo is MUCH brighter but is potentially dangerous and should only be used if you research the proper use of them. See HERE
for battery care.)
JST connectors for connecting the pants LEDs to the hoodie
16-20 awg black and red wire. The gauge isn’t really all that important as long as it is flexible. If you have an old computer you don't use anymore, you can take the power supply and raid it for wires.
Optional but very helpful:
Helping hands for soldering