You can spend as much or as little as you want on a party OP.
You don't need to make "traditional" party bags... Why not go to a pound shop, see what's in season... There's sometimes bubble sets, swords, colouring sets... Loads of ideas.
Take home gift: £1... If you're feeling brave, wrap them, get some shredded paper and create a "going home lucky dip"
Games... No matter what, there will be overexcited children.
In your space, you can hope for good weather and so use of the garden, or, party games...
Musical statues,
Dead lions
Musical bumps
Pass the parcel (pop something CHEAP in the layers) I don't like sweets, I usually include a pack of crayons (4 pls for £1 in pound shop!)
The centre of the parcel? Again, look in sales, £1 shops (they are freaking awesome!)
Treasure hunt (hide things in the room in plain sight that you challenge the children to find)
"Eye spy"
Alternate a moving game with a static game.
Prizes for littlies can literally be a sticker on their front.... This also helps you to ensure that everyone wins something.
Food.
Depends on your expectations. You can feed them each a "picnic on the carpet"
Each child needs a small box/paper lunch bag, card tray... For a small number you might be able to get these at a greengrocer
Make each child a sandwich, carton/bottle of water, piece of fruit, treat item etc.
Lay a picnic blanket/any blanket down
Sit the kids on the floor for a picnic.