I have found connecting devices via crossover cables can cause problems due to LAN speeds not being negotiated correctly. I would try setting the LAN cards on both machines to say 10mb/s half duplex or get hold of a cheap hub. A cheap hub should cost less than PC world charge for xover cable, 15quid ish.
The cable must be working in one direction as your are picking up packets from 'kids pc' on your PC. If you reverse the cable & same happens then cable should be OK & probably a duff LAN card.
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