Shared, transparent, on the record
Both parties record expenses against the allowances in the court-approved settlement, attach the invoice, upload proof of payment, and see exactly what is due on the first of each month.
Each child has their own allowances and totals, so a minor and an adult child are never mixed up.
Label each invoice against the allowance categories in your court-approved settlement.
Entries stay open until the last day of the month, then the month closes with a settlement summary payable on the 1st.
Invoices, proofs of payment and the settlement agreement itself, filed in folders and visible to both parties.