What settings to use when taking outdoor pics of a 1 year old??

I am new at taking pictures (did it many years ago). A friend of mine wants me to take some pictures of her, her husband and their 1 year old. They will be outside at about 5:15pm, what is a good setting for my Rebel XTi? I know very loaded question, should I leave it on the setting that decides for you (green square) or attempt to change the f stop or aperture (AV or TV). I am very nervous if you can't tell, she is a good friend and keeps telling me she has confidence in me! EEEK! Oh I am using the 1.8 50 mm lense.....

I have been using the Av setting most of the time (when a flash isn't mounted). I select the f-stop I want based on the DOF goal. Then adjust the ISO to get enough shutter speed, which is not usually a problem. If the ISO has to go too high I have to rethink my plans or use flash.

Oh, I never recommend the auto-green-square mode. Even P mode gives you the ability to shoot RAW and select your ISO. You can even adjust your f-stop/shutter in unison to get the desired action stopping or DOF you desire.

If you aren't familiar with settings, I'd choose Av, also.