Don't purchase the tickets through a travel agent, including especially OLTAs.

Have you checked what's required for a 13 year old to enter the USA and Germany alone? Can these kids handle passport control, customs, etc. all on their own? You can't expect airline/airport employees to help if you haven't paid the UM fee. Also, what if there's an overnight diversion to a third country? What would the kids do and can they handle it alone? In fact, do they speak English well (under stress) if they're originating in the EU? In the USA, you risk having the kids put into the child welfare system (protective custody) if they're found traveling alone without a responsible adult who has permission from the custodial parent(s).