Packed lunch type meal is an excellent idea.
People will take photos with Santa on their phones so create a photo wall/backdrop/ nice corner scene. If you have a large cloth, a sponge and red paint you can make a brick wall. Add a wreath, Christmas tree and decorations (your own or someone else’s or from the school) and maybe supply someone’s armchair for Santa. It doesn’t have to cost much to put together.
Before Santa comes (he’ll need a grand entrance) organise a sing-song to set the atmosphere.
Print off some Christmas colouring pictures and beg or borrow crayons for every table to keep kids in their seats and focused.
Santa can give every child a token present. Don’t overspend on this part. I’d look for something generic rather than girl/boy, or split by older and younger dc. Craft kits are pretty universal. Or just give a special bauble for the Christmas tree to hang where Santa will see it on Christmas Eve. Or give a sachet of reindeer food (no glitter please)
If you have names of attendees in advance you could make a giant scroll, or several with a nice list, or have a big book where the kids could check for their names. Obviously be very careful to get them all in!
As people arrive give them a name sticker.( If you want to be super efficient you could colour code these by age categories to match gift ) so that Santa can call them by name.
It would be a lot of work but if you got some information for each child (age, pet name, teacher’s name, something they’re good at) you could really personalise the Santa experience. It would take time and effort but not money.
Hot tea and coffee and a mince pie would be lovely. You could serve hot chocolate. If you make it in a large pot and label it out punch bowl style, it’s easy to do. Add mini marshmallows and a candy cane stirrer. Dilute with milk to cool if serving to kids.