No kids exception

My FH and I have already decided that we will not be inviting kids under the age of 12. I have a very small family, so I would like them all to attend if they can. My only cousin on my moms side would have to fly in for our wedding and she has two kids.

My question is: would it be okay To give her an exception to the no kids rule if she needs it because she is traveling. I assumed that I would address her invite to just her and her husband, but if she asks if her two kids could come I will say yes because it isn’t fair for her to have to try and find a sitter in a different state. This would be the only exception to the no kids wedding because everyone else lives within 30 minutes of the venue. Or would this just cause too many issues? We really don’t have the space or $ to pay $80 per kid, and there is no kids menu. I also cannot ask her daughter to be a flower girl and have an exception made that way. My niece passed away recently and she would’ve been my flower girl - seeing someone else in the role would just devastate my entire family and especially my sister/MOH.