At 5, a bouncy castle and some quiet activities (we did colouring / sticker sheets on a table away from the music) is fine. Here's what worked for our 5 year olds:
Hire village hall (£50)
Large bouncy castle with slide (£100)
Teas / coffees and a couple of tins of naice biscuits for mums and Dads (£5)
Kids' food - lunchboxes. I bought some 'Happy Meal' style picnic boxes online (about £15 for 35) and in each one I put:
One ham & one cheese sandwich (white bread as few kids this age will eat brown, cut the sandwich into a star using a small cookie cutter which also removes crusts as no one eats them anyway)
A packet of Pombears (Aldi own brand)
A 'Frube' (also Aldi own brand)
A 'MiniRoll' (also Aldi own brand)
A box of raisins.
Carton of apple juice
Obviously adjust for dietary requirements.
I then did an 'ice cream station' which doubles up as an activity. Buy some paper ice cream cups and wooden spoons (cost a few quid). We had couple of tubs of cheap vanilla ice cream. Each child gets a scoop in their cup and then I set out a table with various sauces and toppings. Dead simple, pretty cheap and they all loved it.
For going home, slice of cake, one of those big balloons with an elastic handle that you bash around and a small sweet cone (buy the cellophane bags online and use various Aldi own brand sweets to fill - small ones like jellybeans at the bottom and some bigger ones on top to bulk it out).
For music, I just plugged my phone into the hall's PA system but a decent Bluetooth speaker would suffice.
We didn't bother with games as it gets a bit chaotic with a whole class. Too much emotion and they get bored waiting for their turn when there's 30+ of them.