My tip would be don't let the grandparents be in charge of the kids squash. Perhaps that was just my personal experience 😂
I did a whole nursery class party for 4th and a reception class party for 5th.
For the 4th party I had 15mins to set up. You read that right. 15mins. By myself 😱
I borrowed a pull along trolley thing (like you see kids sleeping in during the day at festivals as parents drag them around, no idea what they're called) to ferry stuff from the car to reduce trips back and forth.
Shoved up the banners first and threw table cloths out in tables.
Laid out the paper plates and cups next with napkins.
Know how many tables you need to set up and pack for those, pack table cloths on top as you want to get them out onto the tables first.
Unwrap the cellophane from anything wrapped like paper plates cups etc in advance before the party!
Party bags were all prepped night before and loaded into a grab box.
Balloons, pump, masking tape, wrapping ribbon/string and scissors were in another box.
Sandwiches all made and frozen the week before 😂 thawed for the party on reusable platters.
Crudites all chopped the night before and shoved into food bags for pouring out quickly onto platters at the party. Likewise melon and strawberry and grapes.
Shove packets of pom bears into bowls.
Likewise chocolate fingers or animal biscuits or whatever sweet treats your doing.
Cake - everyone wants some, noone wants to wait. Have a cake to sing HBD to and then distribute a fairy cake or something for the gannets to eat while you cut the main cake. Decide how you're going to cut the main cake in advance so you don't have to do the slice maths on the day! Shove the slices in party bags as they leave.
Leave blowing up balloons to be the last job - you can get other parents to help as they arrive, some will be willing.
Don't forget the bin bags.