My degree is in Music Special Ed. If this is for Classroom Music, most music textbooks have instructions for special needs students in the outlines of teaching plans for each lesson. I'm not sure why the songs must be slow-to what end? Kids, all kids, love all different kinds of Music. If the purpose is to teach Music, then we attempt to have the student take in information in as many ways as they are able, i.e., sing, listen, move, play, visually see Music--as they are capable. If the Music is for the purpose of moving with with Music, Folk Music can be terrific as it often has repetitive lyrics & melodies, and can be simple to understand.
   Much of the time, I would simply sing unaccompanied and let the students follow my lead. Beginning with hearing the song, then, adding more activities or ways to "take in" information about the Music. One could have the students listen to the song. Another time, move to the song. Then learn a phrase or more till they know the lyrics. Then act out the song. Then use visual aids to identify the melody moving up & down or moving by twos or threes. Then perhaps play an instrument during particular portion of the song. & so on.
    By doing Music activities in such a manner, one may accomplish learning some of all the Musical basics in a successful learning experience. The special needs student will literally increase their ability to learn.