My cost cutting tips:
Also be very clear that the birthday invitation is for the named child only & not siblings. The wording can be 'Due to restricted numbers at the venue, we can only accommodate the named child etc.'
I've always given 3 weeks notice, the standard here is 6 weeks notice. I usually get about 10 people unable to come due to other commitments. Which is fine for me, 20 kids are easier to manage than 30.
Be clear about RSVP dates especially if it's a pre paid activity. Maybe say if you don't reply by x date then we'll assume you're not coming.
Buy a boxed set of books from the Book People to split amongst the party bags. You'll just need cake & a few sweets. Job done.
Buy takeaway boxes from Anazon or Ebay & put sandwiches, drink, crisps, a satsuma & a little treat bar. Saves on wasted food.
M&S birthday cakes are good value and tasty. No need to spend £££ on three tier -wedding type-- cakes like they do here.
Always look in clearance sections for party bag fillers, prizes & tableware. Wilkos, poundshop, tesco clearance aisle, book people & yellow moon company are all good for bargains.