We are a family of four with six different passports (four US, two EU) . When I travel to the EU by myself or with my two passport child, we show the US passport to the ticket agent when leaving the US (impt. this will be reported to DOHS) and the EU passport in London upon entering to UK which, like Finland, is part of the EU. The passport control line for EU passports is much shorter and moves faster. There won't be routine passport or ID controls within the Schengen area of the EU (continental Europe), so we then put our passports away. When I/we leave Europe for the US we only show our US passports until we are back home. You can really get into trouble entering the US if you are a US citizen but choose not to show your US passport (long story...).
IF I travel with my one passport partner or child we both go together through whatever line at home and abroad as US citizens. After all, we are one family so we stand in line together.... I suggest the girls cement their friendship by staying together through the passport lines wherever they face them.