Children's Birthday Parties

Chance came flying in with an invitation to a birthday party. His first one ever! He has been so excited and mentioning it throughout every day since. This is a big deal his first party invite. I am also rather new at all of this and am a little unsure of party etiquette. We don't know these people that well but I was thankful that Chance has been included.
I have to admit that with this new social setting I have new anxieties. Our unconventional family adds to the uncertainty. How will Chance be perceived and treated as he makes his way though this social scene? While he was included in this round of invites he was passed over by one of hid best buddies in class. We found out that he was not invited to his party. I was and then wasn't surprised. Though they seem to be the closest friends in class I don't believe that this child's mom is all warm and fuzzy for me. If only adults would see people the way a child does this world would be much less complicated.
Where before these issues seemed so far away, they have jumped up on us so quickly. Will Chance be treated differently because of his family? Will he be left out? Will everyone else receive invitations as he is passed over? How am I going to explain this to him and mend his hurt feelings as we face these issues?
The most current issue: what's going to happen tonight at his first party adventure? What in the world am I going to say as the other parents stare at me?