Why would you buy a 26″ ?
A XS <insert brand> 26″ is for the same size as XS <insert brand> 27.5″
Most 26″ bikes even XS sizes have a BB drop designed for 170mm or even 175 cranks which will be way too long. Problem comes when you fit 150-155 cranks you have to put the saddle up an inch and they can’t get their feet anywhere near the ground, you can get round this by putting down sizing wheels until their legs are long enough eg. 26″ wheels in a 650b frame
This is what we did and 150mm is probably still too long to transition off a 24, Jnr is using 142mm cranks at the moment on 26″ wheels on a XS 27.5 frame… though nothing to do with standover* which seems a pointless measurement in MTB’s anyway (other than as a general indication of bike size)… however it makes a huge difference to pedalling (and not causing permanent damage to his knees and hips using oversized cranks).
*I really don’t get the big deal with anyone but a complete beginner, putting both feet down anywhere “not flat” is a recipe for disaster anyway.