Is it grammatically correct to phrase bend your head forward?

Yes, "bend your head forward" is grammatically correct. It's a clear and concise instruction.

Here are some other ways to phrase it:

* Tilt your head forward

* Nod your head (this implies a slight forward motion)

* Lean your head forward (this implies a more pronounced forward motion)

The best choice will depend on the specific context and the desired level of formality.