Six things, one of them good. That is most of the answer — here is the rest of it.
The instinct is to fill the bag. Resist it. A bag with six considered things in it is remembered; a bag with sixteen bits of plastic is tipped onto a bedroom floor and swept into a bin on Sunday. Six is enough to feel generous and few enough that each one can be worth having.
If you are buying rather than assembling, the same rule tells you what you are paying for. A cheap bag is cheap because it contains more, smaller, worse things.
Children rank party bags by their best item, not their average. A single thing with a bit of play in it — a yo-yo, a slap band, a wooden spinning top — does more work than five fillers. Build the bag around it and let everything else support it.
Sweets are expected and children are disappointed without them. The question is not whether, it is how. Loose lollies rattling around a bag with a bouncy ball is a hygiene problem and an allergen problem at once. They belong in their own sealed sachet with the ingredients printed on it, so a parent can read the label without opening anything.
And skip chocolate entirely. An Australian summer will turn it into a puddle somewhere between the warehouse and the party. If you would rather go without sweets altogether, swap them for another toy rather than just taking them out — the bag should not feel thinner for being safer.
A name tag costs cents and changes the object. A bag with a child's name on it is theirs; an identical bag without one is a handout. It also stops the argument at the door about whose is whose, which any parent who has run a party will recognise as worth the money on its own.
Between $8 and $17 a bag is the honest range in Australia. Under $5 buys a bag of landfill. Over $20 and you are competing with the present the child actually came to give. Most parties land at around $12 a head, which for fifteen guests is roughly $180 — about the same as the cake and the venue hire combined, so it is worth being deliberate about.
This is the one people get wrong. A bag that would thrill a seven year old can be a genuine choking risk for a three year old, and a bag pitched at a four year old reads as an insult to a nine year old. If you are ordering for a mixed group, pitch to the youngest for safety and add one item with more play in it for the older ones.
Assembling fifteen bags takes a couple of hours, two shopping trips and more money than you expect once you have bought the bags, the fillers and the sweets separately. That is the actual comparison — not the price of a ready-made bag against nothing, but against a Saturday night at the kitchen table.