Are these the same fares?

Child discounts are typically written in the fare rules, and the portal should be reading the rules and pricing appropriately. Have you read through the fare rules (on portal) to see if the child discount applies?

Is it possible the fare LY shows directly is different than the fare filed via GDS, and therefore potentially different rules?

Just IME, the 5x for booking on portal are rarely worth it to make up for the disadvantages of using it vs. booking direct - I only use the portal to use VentureX credit, and then not again until the next year.