1. Login to your child's gmail account.
2. Click on the gear (top right corner)
3. Click on Settings
4. Click on Forwarding and POP/IMAP
5. Under Forwarding: click "Add a forwarding address" button and follow the instructions.
Once the forwarding email is set and while you are still on the Settings screen click on Filters.
Scroll to the bottom and click on "Create a new filter"
While in the filter window:
Enter your child's email address in the form field. Click on "Create filter with this search" (bottom right). Select Forward it to: and pick your forwarding email address.
test it and you are done.