I used to use threaders, shop bought or fuse wire and they work fine but they are fiddely and usually break easily or your son uses them and loses the bloody thing.
What i do now is when i break the end tackle down i leave a length of line in the tubing and tie it back on itself, if starting from new i thread 25lb sea mono through which flys through. Then when i want to thread my reel line through i superglue it to the line running through the tubing and pull it through, works every time. 1 drop of glue sorted, and i can find the glue in my bag a lot quicker then a threader.