Hi CLP
It sounds wonderful what you are going to do

Everybody loves christmas parties and to invited to one is always really special.
As you are looking for ideas, I think a definite starting time and end time for families with little children. I would even have a visit from Santa at a stated time as well. Santa could give out little lolly bags to each child or even homemade biscuits etc.
Then, you could have on the invitation that a barbecue and champagne party will begin afterwards. That way, people can decide if they want to do one or the other or both.
As for the menu, we always do a barbecue. Its the easiest, encourages people to go outside and salads are so easy. Sounds like fun and a great way to make new friends. Let us know what you decide to do and make!
Mrs B