Do they have a single ticket US-AMS-SOU? If so bags should be checked through and the MSC (Most Significant Carrier's) baggage rule should apply to entire journey.
This could be different when they check in for their flights home, but outbound they may be OK. This assuming one bag per person not more than 20kg.
Last edited by ExXB; 5th Nov 2012 at 16:21.