Socks don't need to have FTC labels, has nothing to do with CPSIA labels. There haven't been any exceptions made for any children's products as to CPSIA labeling. But you could use a hangtag with the labeling information on it, as labels on any socks would interfere with the use, (which is one of the only allowable changes to the permanent rule) in my estimation.
As to Sharpies, I believe the ink washes off. If you need them for fabric items like garments make them yourself on your inkjet printer, use a date coding like I do, JFMAMJJASOND12, some people use W S S F for the seasons, and take a few stitches under the date corresponding to the month and year or season.
As to buttons, check with the button manufacturer. I think Lansing (which makes a ton of buttons sold at Joanns), will give you testing information, if you give them the item numbers. But they don't have to test, and they don't have to give you any information if they don't want to.