Same Gender Taxation

We are in the process of looking at offering same gender health benefits.  If the couple is not legally married then we will be taxing the premium.  However we are not sure how to set it up and have some of questions.

is there anyone who has done this recently that would be willing to discuss this with HRIS analysts who are setting it up? 

Also can you tell me how you determined taxable contribution amount?  We are looking at using COBRA premium.  But are unsure how you account for legally dependent children and non-legally dependent children. Do you have a taxable premium if they have both legally and non-legally dependent children on their plan?