It depends if they have a similar protocol as the British do.
In the UK, the queen can not travel with her direct heir, Charles. It doesn't apply further it seems, as Charles has flown with his sons, and Will with his. Despite the fact that Elizabeth has three other children and now countless grandchildren, great grandchildren and several hundred other heirs.
If the same applied with the DRF, despite her other daughters, Juliana could not travel with Bea. Did Bea ever fly with WA, or has he with his daughters since he became king? There in may lie the answer.