Pulling your navel in will encourage your lumbar spine to flex and your pelvis to tilt posteriorly (tuck). This means if you want to challenge the transversus abdominis, which works in a neutral spine position, but not a flexed spine, then “navel to spine” is not an appropriate cue.
Instead cue to keep the tailbone/sacrum heavy on the table or carriage. Inhale into your back which will anchor the back ribs onto the table and allow the exhalation to connect the front ribs which will engage the upper part of the abdominal wall.
Feel at ease in each class you teach