Wenis - is this your pfb?
Just wondering, because with my first, at 3 parties and the whole thing just passed her by... but with DD2, it is a totally different ball game. She is coming up for her third birthday and already has been to lots of birthday parties (mostly friends with slightly older children - about 4/5), but she knows full well that you dress up for parties, get party bags and the whole shebang. There is no hiding any of it from her because she has experienced it from taking her big sister to parties and collecting her (and lots of crying that she doesn't get to stay too!). Certainly, at 2... my DD has been talking about her birthday party for months now, whereas DD1 didn't even have a concept of them!
And of course, when there are parties it is inevitable that some children will not be invited. I think the problem is often worse at the preschool age, because at least at school there are 30 children that are to be invited and that's it. Whereas you want to invite less at a preschool party (because the mums come too - so you do need to find a room that can accommodate double the number of children) and they mix with different children all the time, there aren't fixed children at preschool because different children do different days... and also the child may be in more than one type of childcare setting - my DD2 goes to both a child minder and a preschool for example, so has two sets of children to invite. You simply can't invite all of them, but so decisions have to be made!
That said, I find the solution relatively easy - not to have the expectation they will be invited. If ever my DDs have said they've not been invited to a party, I've asked them 'are you best friends with X', usually the answer is no - and then I say, well why would you be invited then? The girls themselves know that on a limited budget, they have to choose their closest friends to their party and that other boys and girls have to do the same. So, they know that if they're not best friends / close friends with that child it is unlikely they will get invited, but see it as a bonus if they do. That way, we've never had upset about not being invited to parties... even though it does happen because my children do not have the expectation they will be invited (unless of course, it was a close friend... but thankfully, their close friends have always invited them, so we've not crossed that bridge yet!)