DD1 turned four this year also. We invited all the girls from her kindy class (so 10 others all up and only 8 came). I couldn't do the whole class ... way too many kids. Inviting all the girls (or all the boys if you have boys) means there is a good way to divide them up and not leave anyone out unless you go really small and most kids don't get invited which also works well. I think the big issues come when most of the kids get invited and only a handful get left out. Then it's just cruel but if she only invites a couple of kids that she plays with regularly or all the girls, it is easier to explain and it is not really leaving anyone out.

We didn't know the parents either but I wrote on the bottom of the invitation that we would like them to stay for the party just to let us know when they RSVPed as well as if any siblings would be attending also. Only 1 sibling turned up that wasn't a toddler or baby so it wasn't overrun. There is no way that I wanted to be looking after other people's kids when I hardly knew them but it is a risk because you really don't know how many extras you might get if they bring siblings too. One of my friends had a party and didn't think about siblings - invited 10 kids and ended up with over 20 kids. Turned out to be a bit of a nightmare. But I think if you don't specify on the invitations what you expect, everybody has different ideas about bringing siblings. Something to be aware of anyway ...

DD's party was a great way to get to know the parents of DD's friends and now when I go into take her to kindy, we are all very chatty. It's great!