I humbly offer my own perspective as the child of divorced parents: it’s just as hard for us to let go as it is for you. I don’t presume to speak for your girls, but I know – for me - that it was hard to go back and forth between houses. I was missing stuff too. I had to let go of one to embrace the other. 

When I was really young, I was somewhat convinced that the minute I left one parent’s house for the other’s, the left-behind parent immediately started to party and have all sorts of fun without me. But once I realized that I was missed, just as much I was missing, that’s when I started to feel better, oddly enough. I suspect your girls know they are missed and that, I know, makes all the difference. 

(Yet another post I've told my own mother she had to read!)