Ok — just so you know — my middle (and only my middle) granddaughter also sings on the toilet. But only when she has to pooh. However, she will sing on ANY toilet when she has to pooh. Even if that toilet is in a public restroom located in a Target. Her Aunt & I once stood outside the stall door helplessly waiting long minutes for her to finish. We weren’t sure if she was singing until she finished poohing or poohing until she finished the chorus. But wait we did until she was good & done. :) She is 6 now.
Your granddaughters are not weird at all. Perhaps they just all come from the same planet of origin. ❤