I think it totally s***ks for them. Honestly. They are obviously left out and know it. It is cruel.
We got through it, I actually didn't even hear much about parties in 5th grade when he was 10. Now he is beyond it.
But it still stinks. If there is a party that our kids our left out of and know about it, try to give them a special day doing fun and special things. It helps them a lot, I think.
I think the parties have stopped now but luckily he didn't care much when he was younger. He has a few close friends that he plays with all the time and that's all he really wants to do anyway. He's a pretty tough kid and sticks up for himself. I don't necessarily like the fact that he can have a nasty mouth but I view it as an asset at times.I have had this happen so many times to my oldest daughter that when my youngest daughter has parties, we invite everyone in the class. I would hate to have one child go through that hurt they feel when everyone else is invited but them and then the kids all talk about it in school.I'd tell them the same thing you'd tell them if they didn't have ADHD. Granted, it's never easy to explain things like this to kids. My heart aches just thinking about how awful it feels to be excluded like that.
Think of it this way, they'll be excluded from something for some reason at some point in their lives. Even if the reason for the exclusion is ADHD, you don't have to make a big deal about that part to your kids. You can't change the past and you can't force people to accept your kids, but you can teach your kids how to work through this kind of thing.
Give them both big hugs for us...
I think you did the right thing. We have had this happen too, but after I thought about it for a while, I realized this. If Will was invited, I would sit and worry about what he was doing or worry if he was even having fun. This way, he's always home and I always know what's going on. He never really seems to mind because he's not comfortable in crowds to begin with..That is right imcbmcc! I think this can make our kids stronger and perserve. They all do find their way and succeed. I think it makes them more compassionate to others in their adulthood due to what happened in their childhood, which isn't such a bad thing. The world could use some more compassion in it!
I as the parent of two boys with ADHD (which is managed pretty well) they both recently have been excluded from which I thought were good friends, birthday parties. One mom which I thought I was friends with totally lied and said that they would wait till the following weekend to to something with her son, Jacob and my oldest. Then to find out that they had a "tailgating football party" and he didn't meet the coolnest factor I guess. To me that really isn't a true friend. Maybe that parents can't stand my son but WOW was my son hurt. What do you tell him? And do I let him go when and if they decide to go to the so called movie for Jacob's birthday??? My other son who has many more problems but can truely behave wasn't invited to one of his best buddies party. The child, Levi, asked his mom if my son could come and told him "NO". I don't understand. This past summer we were good enough to give Levi rides to baseball games and watch him if needed but now......I feel hurt myself and really breaks my heart for my kids. They didn't ask to be born with ADHD and they really have coped well with not being invited. But what do you tell them when this happens again, cause it will?? If I were in your shoes, I would call the parents and ask them why my child wasn't invited. Put it back in their lap. This also portrays to your child that you, nor them, will put up with unacceptable behavior. I think these other parents are wimps for not being straight-forward with you and they need to be put in their place. I wouldn't want my child to be friends with someone who has wimpy, chicken-sh** parents.You know what is so sad about this birthday "stuff", is that it goes on ALL THE TIME! I know we went through it with my son, the one with adhd. All of it. BUT, I also wonder if non-adhd'ers also go through it.
Our kids are so targeted that this is just one of many exclusions that us parents have to deal with with our children. 
I blame the parents, who should know better, and teach their children compassion. Shame on them.
Unfortunately, their children learn from them.
My son was excluded from his best friends party, and the mother was a total nut. She rsvp'd verbally to me and then denied it to our party! She did NOT invite my son, who found out because all the other boys were invited and talked about it at recess. My poor son asked his best friend why he wasn't invited and the other boy said because there was no room for him.
This mother thought my adhd son was a problem. Even though her son, I swear adhd also, destroyed the boys bathroom with four others. Mine wasn't involved!! 
Needless to say, she took both her sons, who both got into lots of trouble, and ran away to "find god" as one of the son's told my daughter. This mother is like all the other mothers that we deal with, the ones that think "NOT MY KID" and blame ours!! 
Long story short, the children came back into the school system, and still have "issues" - my son's former "best" friend is now hanging out with total trouble makers. So not much has changed in their life but I am sure they are blaming others for their own children's actions.
Mine on the other hand is an honor student, well like by teachers, not a social butterfly, but good all around kid.
Hang in there. I wish I could offer a solution, but I cannot. Please take comfort in knowing you are sooooooo not alone!! 