"Chúng tôi là những đứa trẻ."
Translation:We are children.
They represent two difference sounds. ch is for /c/ (nearly like /tʃ/ but less aspirated) and tr is for /tʂ/.
In almost Northern dialects, people pronounce these two digraphs the same while in almost Central and Southern dialects, people do distinguish them. The voice for this course is in Hanoi dialect (a Northern dialect). So you will here them the same.
những đừa trẻ is variously translated as babies, kids, children. That's fine but annoying when I guess the alternative and get marked wrong. forces me to do more revision, I guess... ̣I used 'babies', correct answer was 'kids', which is actually colloquial, literally -baby goats!!! Still it's a great app