You don't say what your neighbour's son was doing........
If the LED on your Xbox used to turn yellow but now remains green then it's likely that you installed SID without shadow C protection (one of the main reasons you SHOULDN'T use SID is that it gives you that option), and your guest has run a game that has updated the dash on C and in the process overwritten your softmod. If that is the case you need to start again.
If the LED is still yellow then your guest has simply deleted the dash from E:\evoxdash.xbe (and maybe some other files as well). You can fix this with a boot disc and FTP.
Best next step is to try turning on your Xbox with a (previously working) boot disc in the DVD drive, and see if it boots to the boot disc menu. If it does your softmod is intact and can be repaired, if not then your softmod is history.