It is okay to teach this with the base knee being bent since they have to land their leaps with a bent knee per the compulsory routines.
I disagree that an arabesque is soley a flexibility skill, it requires tremendous strength in the lower back and butt too!
Its all about the ballet bar work, An exercise I like is to have the kids stand in first position facing the ballet bar with their chest against it, point their foot behind them and keeping their hips down and square slowly lift the leg up the back untill they feel a 'pinch' in their butt. You can also do this exercise lying down
My top gymnast has one of the best arabesques I have ever seen, Here's a picture!
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You see how the chest is up and the back is arched? This is correct, a lot of kids dip their chests to lift their leg higher, try to prevent them from doing so, as it does not strengthen the muscle