If you would not like your kid to login unless he finishes his home work or want to fix just 2 hrs of day for him to use computer, Windows 7 comes with an out of the box features which allows you to set Time limits or blocking user for some part of the day, for any account except administrator accounts.
- Go to Control Panel\User Accounts and Family Safety\Parental Controls
- Select the user account which you want to limit
- Set Time Limit for which hours of the day user is allowed to use the computer. Blue indicates blocked time.