I don't know the complete situation either Heather, but I do know that my girls would rather ride with their dad just about anywhere, and going to work with him is one of their favorite things to do. And we've never argued about it. So, it's not like they feel obligated to go with him. There's a term I've heard, "Disney Dad". Dad is just more fun, when he has time with the kids, they PLAY. They don't clean the house, make meals, have a strict bedtime. They play. Kids like to play, so I let them. I take the time to do stuff for me. I relax, sometimes I clean, sometimes I just sit and watch TV-which I NEVER do these days.
I know it's hard when he is playing them against you, impossible even, but remember they love you. You mend their owies and broken hearts and feed them and care for them. They will know this.
I loved the sleepover idea you had with S4. The girls and I would have movie nights on Fridays and rent movies and make popcorn and stay up late.
Maybe we can learn something from our Hs and try to play a little more, but then, who would clean the house?
Each experience in life has formed me, become part of me, made me stronger.