I've never been in your exact situation but I'm a stepmom so I know how it is!
The best thing you can do is not try to appear to control things with the dad. And only fight the battles that need fighting. It's SO much easier when you're on good terms with the father! We used to have a horrible schedule, she was every other day and mom or dad's and she hated it and it was disruptive. Well, we stayed on good terms with the mom, and now we have a much better schedule to our liking. It happened slowly. So you might have to "go with the flow" for a little while. Don't worry, it will be okay. (But do keep documentation...we do)
The other thing is please PLEASE help your daughter to HANDLE the situation. Be on her side...because no one else is. But neither one of you can help the visitation schedule so teach her how to deal with it. I know it's heartbreaking (our 8-year old sometimes cries and begs not to go to mommy's and it breaks our hearts) but we try to help her handle it over there. We empower her. We give her suggestions on how to deal with things that come up. That's the best thing we can do for her.
The last thing, my stepdaughter is 8 and we got her a cell phone. We started with one of those with only 4 buttons and she knew she could ALWAYS call us if she got scared or needed anything. That was really reassuring to her. She's never had to use it. It also helped because we told mom that she could (under no circumstances) take the phone away AND we might call unexpectedly and it needed to be on! It has helped us to know that she will call if she needs anything.
Also, whenever we sent my stepdaughter to mom's house with a toy or clothes we never saw them again. So I do advise that your children NOT bring lots of toys over there unless you make them responsible for bringing them back and you're okay with the possibility of never seeing them again. Send them in clothes you're okay with not seeing again as well. They are kids, and they probably won't remember to bring stuff back. We spent a lot of money on nice clothes that disappeared...until they were too small for her! Now I send her in either thrift store stuff or stuff from mommy's house.
HUGS you can do this!