The cheapest way to do it would be to right click on the desktop's ethernet card, go to Properties, and select "Allow other users to connect through this computer's internet connection"
However, your daughter can only access the internet when your computer is turned on. A better solution would be to get an inexpensive wireless router. You can get them for $10 or free after rebate if you watch the Best Buy/Staples/Office Depot flyers.