Bridesmaid dilemma

My hubby to be wants his 5y old niece to be one of our bridesmaids. I’m not sure what to do. I don’t get on well with her parents (my brother and sister in law), there’s no room left in the wedding car now, her mother (my sister in law) would have to bring her to the church and get her ready as I don’t think she would stay with me on her own as she’s very shy. I just don’t see the point? Plus her parents were even going to bring her as they wanted a child free night away! My hubby to be thinks it’s nice as she’s family and gets on well with our little boy who is page boy. I just don’t know what to do