I'd tag post #2873556 with spread legs.
Legs apart is supposed to be for characters in a standing position or similar with their legs apart. This is a completely different posture to most of the sitting position images that you've been adding the tag to, and many of these are perfectly covered by either wariza (post #2872557 and children are basically a textbook example) or spread legs, or else are squatting pictures.
You could argue that it could be named differently, with the legs apart tag then used as a catch-all tag for all situations in which the legs are visible and not together (do we really need this though?) but what else would the current legs apart be named? You can't really do something like standing with legs apart as the tag encapsulates more than just standing pictures.
Not to mention that if this were to happen then the best things to do would be to change the tags before you start adding new pictures, as otherwise this just creates noise and makes the change much more effort.