Make separate totals for money and promises
Write each person's offered amount next to what they have actually contributed. Three $30 offers make a $90 plan. Two received transfers make $60 available. A household can plan around the third offer without spending as if it were already in the organizer's hands.
Decide what can be ordered today
Suppose $45 of reusable supplies and a $20 decoration order are ready. Buying both would require $65, which exceeds the $60 held. Buy the $45 items and wait on the decoration, choose one costing $15 or less, or ask someone to cover an agreed advance. Do not leave the organizer to discover the $5 gap after checkout.
Close advances when contributions arrive
Before ordering supplies, name the person allowed to commit the group to the purchase. If the order must be placed before the last contribution arrives, discuss a smaller order or an explicit advance. Update the record when the money changes hands.
If an adult advances $5, write down who did so and whether the group agreed to reimburse them. When the remaining contribution arrives, pay or credit the advance before treating the new balance as freely available. Save the final list of contributions and purchases until the event's open expenses are settled.