You've messed up already as the first rule of children's parties is to avoid having one if you can help it ;)
Don't overthink it. Children are highly stimulated just by being 'at a party', they are happy to chase balloons around and eat party food. Bouncy castles and entertainers cost quite a lot and might just make kids a bit more overstimulated.
Don't try too hard with food - they will eat crisps, biscuits and cake. Put out celery and carrot for show if you must. Making homemade stuff is barely worth the effort, they want the real junky stuff. Do some pizzas and sausage rolls if you have access to an oven.
Cosy corner with books and beanbags or cushions is good. A table with colouring or similar is good too.
Make it clear if parents are expected to stay and if siblings are welcome.
Have a good understanding of how many kids if you're doing party bags, nothing worse than one or two kids not getting one. Have a back up stash of bags of chocolate buttons or something for random siblings who turn up.
Have a table for presents and cards.
Face painting is a good cheap thing and you can get stencils you just dab over if you don't want to do a full complicated face. Or temporary tattoos are also easy and cheap enough.
Fairy cakes are easier to hand out than cutting a big cake - if it's a big one, have a knife and a plan of how to cut it without many small fingers getting in the way!
The party girl or boy can often be overwhelmed - if you have a spare grandparent/aunt/family friend who can be allocated to look after them while you do the organising bits, it can be a help. Otherwise you might have to either abandon running the party or leave your kid crying in a corner, neither of which are ideal.