You can buy Math World from the 10monkey's Store
NOTE: Your child may already have a user account, e.g. if he or she was signed up in the Number Rumble competition. If this is the case, do not create a new account when activating your license. If you create a new account, then your child’s existing account will not be upgraded. When activating your license, choose "Upgrade an existing account" instead. Ask your child or your child's teacher for the username and password.
To upgrade, you can also log into Math World with the child's user account, and from there press the Buy button. The Buy button may not be visible during school hours though.