Hey Alan,
I apologize, I did not notice you were referring to a baby cham. Koabich is right, in that many breeders are against using 10.0's for babys. I personally, have gotten away successfully with using a 10.0 on a female nosy be from 1 month of age and on with no problems. When observing her, I could see that she would make her rounds under the UVB, then UVA, and then in the plant area to hide. I am speaking on personal experiences. If I were to do it all over again, I would stick with a 5.0, even if the screen is dark, just to be safe....just make sure you can get the baby close enough (about 6inches) to the uvb light so that its actually effective.
I built my cage, and opted to use a 1/2inch pvc coated mesh screen (only for the top) that was recommended by Roo. So in my case, plenty of UVB gets thru without a problem. Im using a 10.0 bulb for my ambanja adult male. The front and sides of the cage are a dark mesh. Keep us posted and good luck.
Jim