Hi Chris, since these are bare wood stairs nailing/screwing from the top obviously isn't desired. So if you have access from underneath, one trick we use is as follows. Get some 2x2 cedar, cut to the depth of the stair tread (probably around 9-11 inches, the size you have from the underside), glue and screw to the risers (liquid nails for glue), then screw through the 2x2 into the stair treads. This will draw the tread down snugly to the risers and hopefully eliminate the squeaks which are caused by the risers and treads not being securely attached to each other. Depending on the thickness of the treads you're going to want 2-3 inch screws (nothing that's going to go through the tread above, then you're going to wake the little one when you step on a screw tip, LOL). Ooops, I forgot, also apply the liquid nails to the side of the 2x2 that meets the underside of the tread.
Hope this helps, and let me know if you have any questions.