OK, basically, what you're going to do is this:

1. Power up your Linksys router without connecting it to anything.
2. Plug your computer into your Linksys router in one of the LAN ports.  Restart your computer.
3. Using your computer's browser, log into the Linksys router.
4. Change the IP address range that the Linksys hands out to a range that the U-Verse router doesn't use.  The U-Verse RG uses 192.168.1.x, so for example, set the Linksys up to use 192.168.2.x on the LAN.
5. Once this change is made, you'll need to restart your computer again to get a new IP address in the new range.
6. Log back into the Linksys.  Set the WAN IP addressing method to DHCP.
7. Plug the WAN port of the Linksys into one of the LAN ports of the U-Verse RG.
8. The Linksys should get an address from the RG in the 192.168.1.x range.
9. Unplug your computer from the Linksys and plug it back into the RG.  Restart your computer again.
10. Now log into the U-Verse RG using http://192.168.1.254.  Go to Firewall -> Firewall Settings.
11. Select your Linksys from the pull-down menu, make sure the IP address you're selecting matches the IP address the Linksys obtained in step 8.
12. Scroll down to the very bottom and select "Set to DMZ Zone".  Click OK.
13. Unplug your computer from the RG and plug it back into the Linksys.  Restart your computer.
14. Restart the Linksys router, it will now get a new IP address via DHCP.  This will be the outside IP address that the RG has.
15. With your computer plugged into the Linksys, verify you can access the Internet.
16. On your computer, browse to http://192.168.1.254 to log into the RG.  Go to the Home Network tab.
17. On the right, click the Disable button under the RG's wireless interface to disable wireless on the RG.  (You will use wireless on the Linksys now).
18. Finally, move any wired computers to the LAN ports of the Linksys instead of the UVerse RG.  Re-set up any wireless computers to connect to the Linksys.
19. Leave all DVR/STB units plugged into the RG directly, do not plug them into your Linksys.

Now you can use any parental controls you need on your Linksys.