The base rule
Pick a family's most expensive active seat. That's the 'first seat,' billed at full rate. Every other active seat (no matter which child, no matter which module) gets a 20% discount applied at that seat's normal rate. The billing engine recomputes nightly, so adding or removing siblings is friction-free.
Two kids, two modules
Family with Lucas (Lingua French, $19/mo) and Mai (Math K-12, $9.50/mo — cascading addon rate). Lucas's seat is the more expensive, so Mai's seat gets 20% off: $9.50 × 0.80 = $7.60. Family monthly: $19 + $7.60 = $26.60. Saves $1.90/mo versus no-discount baseline.
Three kids, mixed modules
Family with Lucas (Lingua $19), Mai (Math addon $9.50), and Ezra (DELE prep cohort seat $25/mo). Ezra's $25 seat is the most expensive, billed full. Lucas and Mai get the 20% discount on theirs. Family monthly: $25 + ($19 × 0.80) + ($9.50 × 0.80) = $25 + $15.20 + $7.60 = $47.80. Saves $5.70/mo vs no-discount.
Five kids, school seat tier
School-tier families (typically homeschool co-ops with their own seats) work differently — the seat tier discount already prices for volume. The sibling discount stacks on top: 20% off every seat after the first family member at the tier rate. A family with 5 kids at the small-tier $17/seat rate pays $17 + ($17 × 0.80 × 4) = $17 + $54.40 = $71.40.
Cascading discount vs sibling discount
These are two different mechanisms that stack. Cascading discount = addon modules get $9.50 instead of $19 within a single account. Sibling discount = additional family members get 20% off their seats. Both apply at the same time; the billing engine handles the math correctly.