Thread for sewing silk? I have a pair of silk pajama pants that have come apart along one seam on the leg.  There’s no damage to the cloth and it’s spreading very slowly, so there was just some flaw and that bit of seam was not fastened properly.

To fix it, what kind of thread and needle should be used?  I’m leery of using the generic thread from a wardrobe repair kit (a couple dozen miniature spools in different colors) without at least checking first.
For a 4 inch repair I would use the yarn and needle in the repair kit, and do it now before it spreads further. 
Use the most matching colour and the finest thread if you have several options, and as thin/fine a needle as you can handle for the repair.

When sewing a whole project, it is good to get good quality thread and needles, but for a small repair you use what is at hand.

While silk thread is available, it is expensive and many people will still use polyester or cotton for the hidden seams and only buy silk if there is top stitching where it shows. When you do buy it for the top stitching you might well use it for the rest of the sewing as well.