I think children and parents will turn up expecting some kind of ‘event’ even if you haven’t specified as others have said. Parents may also discuss this too and ask what others are doing. If they are going and what time. They might all agree to turn up at the same time. This might be less likely in December though for a child in reception as they might not have been many parties already. It depends if it’s a well connected group of parents who already have contact details. How many have you invited? You need to be prepared for this. Lots of 5 year olds all turning up at the same time will need some structure to this.
You are inviting people around. You can say don’t buy presents but for most people this will feel strange and rude to turn up without a present so I’m thinking the majority will feel obligated to bring a present.
I agree that it is very unusual. I was initially thinking SC was late teens or something and had agreed to this. Whilst parents might not expect much with the wording on your invite, I imagine most children would expect a party, party food, singing happy birthday all together, and taking a party bag home. I bet most children will not have been to a ‘pop in if you like’ event. Maybe most parents too. It’s the type of thing you do for adults not children.
I agree that a big part of this for your SC is having friends around and them singing happy birthday and blowing candles out. That cannot really happen here and the blowing out of the candles is often a big thing to children.
I’m also concerned that parents won’t understand it, appreciate it’s not a proper party and be not sure what time to arrive etc. They’ll could talk to each other and agree a take to arrive, but they could also all turn up at the first time you state on the invitation. However the alternative is that they might not turn up at all. What have you told your SC. I imagine you’ve said something. Is he expecting a party? Even if you’ve not said party? How upset and sad will he be if no one turns up or only one or two other children, and he’s been so excited about it all day? Waiting for friends to turn up? If only one or two turn up towards the end, then he’ll be asking you ever five minutes when someone is going to arrive!
If I was you, I’d totally rethink this. You don’t have to do much to entertain them. Have a few spaces set up with different toys (Lego, cars, colouring, crafts, etc - sorry rather sexist list) and have musical statues, pass the parcel, etc. Do a few sandwiches and cucumber/carrot sticks.
I’m assuming though perhaps part of this is you’ve invited a lot of friends or the whole class and couldn’t cope if they all turned up together….