I totally agree with what you've all said. The older girls in the Dutch Annie production were fabulous- very believable and incredibly professional. You're right, that they were all very developed and vocally trained, too.
All of the girls had vocal training of some sort and most had some form of dance training. Some attended performing arts schools and they all had experience of one kind or another. As well as amateur theatre companies, there are children's concerts and music festivals every year in the Netherlands, which many of the girls had taken part in before doing Annie, a national children's choir and performing theatre troupes.
Throughout 2001/2, there was a huge National Tour of The Sound Of Music in the Netherlands (see videos on YouTube)and, as in Annie, several groups of Von Trapp children were cast because of their limited show allowance. Many of the girls who did Annie had also appeared in this tour. Stephanie van Rooijen, the brilliant Annie from many of those videos, was one of the Marta's and another Annie, Kika Wilschut, was a Gretel. One of the orphans, Lizette Hart, played Brigitta and I believe others were involved, too.
The same has happened since Annie ended in March last year, with many girls going on to appear in the same productions. Cherida Krijgsman, the younger sister of the Annie, Cheyenne, who joined the Annie cast to play Molly when it moved to Efteling, joined the cast of Pinokkio, along with other former Annie girls. Her Pinokkio audition video is on YouTube and she sings Hard Knock Life. Other girls have moved on to Ciske The Rat.
Many of the girls have YouTube accounts so search for them. I know that Annie Kika Wilschut has an account (Kikannie), Molly, Eline Breuking (ElineBreuking) and Cheyenne Krijgsman. Eline has posted a video of the full Een Zwaar Bestaan 'Hard Knock Life' sequence, including the dialogue and the reprise. There are also videos of the project she did after she left Annie, just before it moved to Efteling. She had to leave as she became a panellist on the Dutch version of Are You Smarter Than A 10-year-Old? (US: Are You Smarter Than A 5th Grader?)