Nice thing to do but you already pay your musician friend for recording only backing tracks, did she help you with any of the writing of your songs lyrics/tunes, is she giving you back the money you already pay her, also who owns the copyright to the nursery rhymes.
If she only recorded backing tracks and you both agree £5 per song and she was pay then as far as i know she not entitle to any percentage, also you do know more than likely you own no rights to those nursery rhymes.