These are the 'elementary' ones, i.e. nothing from the technique of scales, arpeggios, fast or extended repeated notes, thirds, sixths or octaves. At most one thumb turn in scalar material, at an at most andante tempo, is present. They're supposed to be in some semblance of an order, but they're probably give-or-take-15 in either direction. Hard to compare the long ones to the short ones, as well. I'm assuming that people are willing to simplify ornaments as needed, when it seems plausible for the passage.
Also, there are definitely some typos in the list above (e.g. K. 14, K. 415, didn't keep track of others). Just posting since this list pops up under google searches. Basically, you or your student can go through these before getting to 'serious technique,' so like while working on book 1/2 of Hanon. Way more to choose from than you'd have time for.
These are not necessarily the 'easiest' Scarlatti Sonatas to someone who already has scales, arpeggios and repeated notes down, but there is probably considerable overlap. Asterisks mark ones with some musical worth.
K 40*
K 34
K 32***
K 42
K 80
K 291*
K 453
K 94
K 59*
K 95
K 308
K 81***
K 63
K 294*
K 310
K 109***
K 274
K 287
K 481***
K 285*
K 431
K 281
K 75
K 415
K 471
K 280*
K 323
K 15*
K 88*
K 3***
K 322***
K 279*
K 292
K 177
K 238***
K 60
K 92
K 276
K 210*
K 230*
K 164
K 286
K 151
K 148*
K 35
K 398
K 374
K 267*
K 213***
K 401
K 165
K 206*
K 69***
K 364
K 36
K 117
K 345
K 243
K 304
K 327*
K 362*
Topic: scarlatti sonatas in progressive order of difficulty?