child's shoes, slippers
靸 centers on soft, slip-on footwear: children's shoes and slippers, both light and easy to put on.
靸 combines 革 (leather) with 及, which likely contributes the sound. The character is associated with soft leather footwear.
Leather 革 plus the shape 及 suggests something you slip your foot into quickly: soft shoes or slippers for a child.
For ソウ, imagine sewing soft slippers: sew -> ソウ, and you are stitching leather 革 into cozy footwear.