Neither FH or I want to have a flower girl or ring bearer mainly because we don't have any kiddos that we are super close to. We don't have any nieces or nephews and rarely see our friends that do have kids (they live far away).
FH's cousin has asked us twice now if her daughter can be a flower girl. Her daughter is 12 and will be 13 at the wedding. In my opinion, that is way to old to be a FG and we really don't have a special relationship with her. It's also an additional cost that isn't worth it to us.
How do we politely explain that we don't want a flower girl or ring bearer? She doesn't seem to be taking no for an answer.