Family Drama w/ Children

Hello! I need some advice regarding some wedding drama...during this past Christmas we were with his family and his nieces and nephews were not the most well behaved. There are 8 of them total, 6 of them are ages 11 and younger. A 15 month old was literally scaling the blinds and her parents were letting her, while another child trashed a bathroom with makeup (nothing was done to discipline her). None of these children understand the word "no." One of the 4 year olds refuses to use "please" & "thank you," and instead spit in my face when asked to use his manners. At another point, one of the children threw his head back into a cake (because he didn't get his way), and then had frosting in his hair, and his parents didn't wash it out so he was getting frosting all over the furniture. At another point, one of the nephews was throwing candy across the aisle at church and his parents were laughing at him instead of correcting him. The list goes on and on, but after Christmas, my fiancé and i decided we were going to have to exclude all children from the wedding day, so that it could be drama free and we didn't have to deal with the behaviors. We told his siblings and parents our plans, and they seemed to agree. (We just told them we wanted it to be an adult-only affair...we didn't get into the issues we've had with the poor parenting and the resulting poor behavior from the children). We had a plan to help them find sitters for the day. We thought the issue had been taken care of, and we have made wedding plans to have nicer chair covers, a lot of glass, and overall just a lot of non-child-friendly decorations. (Also, side note, fiancé's sister had a child free wedding herself). Two weeks ago, we were asked by fiancés mom if there has been "any give" to our no child rule, because "the little girls want to experience this princess day, and it would just be so much fun for them!" We also found out that fiancé's sister is planning on bringing her youngest with her to the wedding no matter what (she will be pushing 2 years old by that point). Apparently the soon to be in-laws aren't happy with the no child rule, and they are pushing for the kids to come. This would require re-planning SO many of the decorations because I can't trust how the kids will behave the day of the wedding. I'm frustrated that our requests aren't being respected, and (not to be mean), his family is not paying a single cent for this whole day, so I find it rude that they are trying to dictate the guest list. I don't want to be steam rolled, but I don't want to cause any rifts. Any ideas for how to deal with this?