I tend to agree that classical music is NOT soporific, but dozing audience members across the world would suggest otherwise.
The first thing that comes to my mind is the piano trios of Haydn. He wrote a lot of them, and you could get a complete set, or at least a couple of discs. That way there would be both calming regularity and pleasing variety, and you would be giving your daughters a marvelous foundation for their music education. They are very profound, inventive pieces, but the beauty of the melodies and harmonies is right there for all to hear, and if they have dark sections, they don't stay that way. I would especially recommend the performances by the Beaux Arts Trio. It is partly the acoustics and partly that they played together for so long that they were unbelievably mellifluous. I especially love the jewel-like piano playing of Menahem Pressler.
Certain other pieces come to mind too, if you felt like making a compilation:
Chopin's Barcarolle (note: the genre of barcarolle imitates the sensation of rowing, or of water lapping on a boat (barca) ... the essence of soothing!):
https://m.youtube.com/watch?v=fuoJVB-waC4
[Schubert, Piano Sonata in A major, 4th movement (NOT the whole sonata, the slow movement is fabulous but chilling):
https://m.youtube.com/watch?v=6TXIBCNqIUw
ETA: Never mind about this recommendation, I forgot about all the inner drama and just remembered that heavenly theme. Such a wonderful sonata though!]
Schubert, the Trout Quintet:
https://m.youtube.com/watch?v=zTk_Eh_L8JA
Mozart, slow movement of the Clarinet Concerto:
https://m.youtube.com/watch?v=BxgmorK61YQ
A Schubert male chorus on the theme of night:
https://m.youtube.com/watch?v=wIyXA1TovaA
Chopin, slow movement of the piano concerto in E minor:
https://m.youtube.com/watch?v=FSSx9Z7-dJQ
Bach, slow movement of the double violin concerto in D minor:
https://m.youtube.com/watch?v=H9V0Pr0aTi4
Bach, violin sonata in E minor (wow, hard to find a complete or good performance on YT... I don't like early music approaches and they're not soothing):
https://m.youtube.com/watch?v=66tE4I6iROM
ETA: Mozart -- a compilation of the slow movements of his piano and violin concerti should work well. Too many to link.