I'm sorry @putthefireonplease - I'm trying to be polite here.- but this sounds like a get together for you, and not your dear 5 year old son's birthday. I feel sorry for him that his mum will be having a nice time, while he is expected to play nicely (and probably quietly) with some possibly random children who come around. Why don't you keep that scenario for your next birthday?
Also, children aged 5 to about 10 years old are the most appreciative of a party, and it doesn't have to be held somewhere expensive either. In fact, I think that many children that age would prefer an old fashioned party, held at home, with games like Pass the Parcel, musical cushions, pin the tail on the donkey, etc. You don't even have to hire a balloon making clown, etc. I didn't when my children were little, and all the children seemed to love it. Of course - unless you live in a mansion - you don't need to ask all of your child's class to the party, about 7 other children would probably be about right.
Then old fashioned party food is also fine, and cheap; eg grated cheese, and p.nut butter, sandwiches (seperate, not together!), crisps, mini sausage rolls, jam tarts, jelly, maybe something seasonal from the party isle at a supermarket, and the birthday cake? The last thing you should probably provide are 8 going home bags (your child will want one too!).
The bags themselves are chesp to buy, then go to one of the cheaper shops and buy 8 small boxes of crayons, 8 little wind up toys, if for girls maybe 8 pretty scrunchies, 8 little bouncy balls, maybe 8 novelty rubbers - erasers - and if you can find some small enough, 8 tiny notebooks. If you also have some greaseproof paper and pretty napkins to wrap a slice of birthday cake each in, to go in the party bags, then that should be ok, and not very expensive.
Although I have said 8 for all the bags, because in my imaginary party that is how many children there will be, I would probably make up 10 or 11 bags in case any of the guests cute younger siblings come with their mum or dad to pick up their children up!
Yes, your DC's birthday will probably be more tiring for you than just having X amount of people dropping in for cake, but it is really worth it to see your small one, and hopefully their little guests as well, really very happy. I'm sure the people you gave the initial invitations to won't mind that you are now having a small party instead.