Well you wont take off without them

The after take off checks are done when the flaps are up and thats when I'd consider the take off phase is complete, not when the wheels leave the ground.

Electronic devices are off for the approach phase not just the actual landing and most of the same risks exist in the initial climb phase as the approach. I dont think initial climb is something that would be understood by many passengers.

When does the electronic device light get turned off on the LCY 318s I wonder?