Hi E..
Man, I have been through this situation before, and let me say I feel your pain. Okay, so this is going to be long, but I feel like I have a lot to share. I've been divorced 4 years and the first couple years it seemed like everything was an issue, but no issue was more heated than the "clothes" issue.
First off, it may be helpful to see what the law is in your state, or if there is anything in the custody decree, just so you know what your responsibilities are and what you’re dealing with. My ex tried to pull a lot of "well, I pay child support so you HAVE to do this or that", but when I would check with an attorney, they’d tell me what the actual laws were and they usually aren’t even as black and white as you think. If it wasn’t written in the decree, then basically it was up for debate, in my case.
After finding that out, I think you have a few options, to sum up what people have said here and to share what I've went through, myself. First off, of course, if they will have a civil discussion with you then it‘s best to talk to them and try to come to a COMPROMISE. Remember, that means it’s possible that neither of you will get what you want. Stay cool. Come with solutions. Ask questions. Use "I" statements, and don’t play the blame game. I actually found that my ex’s wife resented me talking to him, so I would get more accomplished if I just approached her, right off the bat. Maybe that will or will not work for you. Always focus on the fact that it’s your daughter that will benefit from working things out. You aren’t doing this for your own benefit or for theirs; this is for your daughter. You want her to feel good no matter where she is.
But, we all know that certain people just aren’t willing to talk or compromise, they just want to make things difficult, or they are too angry to see a solution. In that case, I would do as someone suggested here; create a wardrobe that goes to her dad’s for her and stays there. Remember, this is your child, and if they are going to not let her wear anything from their house (which doesn’t even make SENSE) when she’s there, then you want her to feel good about the clothes she has to choose from at his house. You can find some nice, appropriate things at garage sales and clearance racks, even hand-me-downs from friend or family with an older girl (my kids LOVE this). Communicate with your ex when you send them that those clothes can just stay at his house, and ask him to let you know (and even bring them back) when/if they don’t fit any longer and you can replace each outfit as she grows out of it. Your best bet is to make this a NON-ISSUE. Communicating with them is the only way that this is going to work. Sure, they have money and they can afford clothes for this child themselves, and that totally SUCKS, but apparently they are unwilling to do this and YOUR DAUGHTER is the one suffering. You have to be the adult in the situation and just step up to the plate and make things right for your kid.
If this still remains a problem, and all else fails, I would take it to a mediator. You will want to DOCUMENT EVERYTHING for a while so that you can prove there is a pattern. You want to be sure that you are coming to the table and trying to compromise with these people in the tough situations, that you are trying to co-parent with them, and that they are just unwilling to compromise. You may end up in court OVER CLOTHES, but if that's what it takes is to have something written in the custody agreement, then so be it.
In my case, I figured out (through counseling) that my ex and his wife were having issues with their own marriage, and so the only time that they could "come together" was when they were lashing out and arguing with me and about me and creating hell for me. Once I removed the conflict and just didn’t allow them to rile me (at least as far as they knew) then everything started getting better, and eventually their marriage crashed and burned. Maybe that’s what’s happening in your case too, who knows? But, the best thing to do, in my case at least, was just never to "lose my cool" with them. Always remain calm. Don’t bite the bait when they try to harass you, tell them you’ll call them back when you’ve thought over something their complaining about if you need to, just BE THE ADULT. E., you don’t have to be a doormat for these people, and let them take advantage of you. Stand your ground when it matters, and the rest, just let it go in one ear and out the other.
Also, like someone else said, this is not something you should discuss with your daughter. I know that it’s so tempting to ask her what’s going on at her Dad’s house with this, but you need to leave her out of this discussion. I mean, if she comes to you, upset about having to wear clothes 10 days in a row, that’s one thing. Be there to comfort her, of course, tell her that you’ll talk to her dad. This is an adult situation that should be handled by adults. I agree with the other people that said that if they actually DID as she said and forced her to wear the same clothes, and they were dirty, then that is considered neglect, so you need to get this straightened out as swiftly as possible so that your daughter can benefit from the calmness.
Nowadays, clothes aren’t so much an issue for us, and hopefully you guys can get there too. Now, we both buy things for the kids and the kids are getting older so they just take things back and forth to our houses however and whenever they want. I do usually end up with most of their clothes at my house, only because I keep them more, but I let them take whatever they want to their dads and bring home whatever they want. He usually goes through things every now and then brings a big "load" back to my house just so that they have enough stuff for school, but it’s really gotten to the point where we just talk and help each other out and support each other however we can, even if we don’t always agree.
Good luck with everything. I hope that it gets better for you. Let me know if you ever want someone to talk to!