Birthday Parties: Invite every kid in your child's age group/class*, if posssible, including children with special needs. If you are unsure you should ask the child's parents what accommodations, if any, might have to be made. Likely, the parent will be so moved by the invitation (as they can sometimes be rare) they will be speechless for a moment. Don't pat yourself on the back, you haven't done a great thing, you've done a human thing. INCLUDE ALL CHLDREN. Doesn't that seem painfully simple. Why am I even typing this?
The parent, depending on the level of disability or the type of condition their child has, will likely attend the party as well and you will have another parent there to help. It's a win/win. It is YOUR JOB to make sure your children understand the diversity in their classroom and to ensure that there is no fear, but instead understanding, of a child with a condition or a diagnosis.
Here is a lesson in empathy for you: Imagine picking up your child from daycare/school everyday and never pulling that elusive birthday party inviation from their cubby/backpack. I know this is about the kids and not so much the parents, but that parent is crushed for their child. I have had multiple conversations with parents whose children are about to leave elementary school and have yet to receive their first birthday party invitation from a classmate. To me that is nothing less than criminal. If you are filling out the invitation with any measure of pity or obligation in your heart, skip it, we can wait until you truly want your child to experience all different kinds of friends.
*If the group it too large you can invite all boys or all girls.
Playdates: I know this one can seem more daunting if you are the parent of neurotypical children and you are unsure of what to expect from a child with special needs. Here's a tip: ASK! After you have taken the time to ask you may find that you know more about the child's particular condition or diagnosis than you might think. You will be able to explain it to your child and with some very simple modifications you can usually set up a very successful playdate.
I know from experience that children with autism struggle to socialize and their parents are often encouraged by professionals to include them in as many social groups/situations as possible to help build on their skills. This is easier said than done, when classmates and their parents are too fearful to ask questions or include a child that might seem a little different than their own. Again, the parent will likely stay and you'll see how quickly children can adapt. Remember, this is not your pay it forward moment for the week. This is a choice to see a child as a child and not a diagnosis to be feared and avoided.