My girls are all teenagers now, but they were young when introduced to 'alternative' lifestyles. For us, we just kept it honest and simple for their level. Hubby and I refer to each other as 'hon'.
So when it had to be explained we explained that they have friends at school who's parents aren't together anymore. Hons that are divorced, or hons that remarry. So they have step parents. Some guys like girl hons, some guys like guy hons. Some guys like both! Some people have more than one hon, some only want one hon. As long as hons treat each other with love and respect and are honest then it's all good!
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year