What do you do when people do not RSVP to party invite?

[Poppssy]

It is my son's 3rd birthday party on Saturday and 2 people have not RSVP'd at all. Problem is that I do not know them. His nursery gave me a list of children he plays with and they distributed the invites so I don't even know what the parents look like. And I can hardly ask a 3 year old if he is coming or not...
Would you cater for the "just in case"? I just think it is so rude when we don't even know each other to not reply. And I would not dream about turning up if I had not RSVP'd. But then again, if they do turn up and there is no party bag or no seat for them then it is the kids that will be punished which is not fair. Unfortunately I have gone over-board on party bags and the catering is per head so it will cost a lot extra to have a "just in case" for what could potentially be 6 extra people (at a party of 25) or even more if they have siblings as well.
What would you do? Any advice welcome.
Lesson learnt is to never invite people I do not know personally again :-)

[Piscivorus]

Could you send a note via nursery saying you are sorry to bother them but could they please let you know if their child is coming as you would hate them to arrive and not have a party bag, etc?

Why is it potentially 6 people for 2 invitees? I always catered for the children only, not accompanying adults or uninvited siblings

[Poppssy]

As the children are 3 I would assume they may turn up with both parents and siblings. I did not specify anything on the invite. I more or less know everyone else that are coming and many of them are coming 2 adults and with siblings so I just assumed that is pretty normal. Either way, without an RSVP your guess is as good as mine.
Trying to get the nursery to chase them down might be an idea. I feel a bit of a muppet getting them to do it but at least it will give me an idea. Thanks!

[IcingandSlicing]

In my experience who didn't rsvp didn't come, but yes I was planning to leave them without a bag in case they come. Thankfully the food was enough to cover, but still I think it's rude not to reply even if you are not coming.