I am in the same situation as you, my FI has three kids aged 16. 13 and 11. The oldest and youngest are boys so he is having them be his co-best men (this way no feelings were hurt with his friends) and his daughter is one of my bridesmaids. We are incorporating them into the sand ceremony. I will have a color, FI will have a color and the kids will share a color ( I felt like 5 colors would be too many). This way we are representing the joining of a family