My son is five. He loves doing Cosmic Kids Yoga on youtube. I am fine with doing yoga but after my weightlifting program and with other restriction I can't really do yoga. I have so many bad joints and other areas- sub-clinical. How do I prevent injuries if he presses me into doing his type yoga (he is a normal five year old so he is very flexible)?
There are usually some modified, less strenuous poses you can take that will help you on the way to opening locked muscles and poses not accessable to you at this moment, or you can use props to modify the height of the floor or get better balance. It's all dependent on the poses and difficult to say without seeing you in action.
As you say, your son is five, and is still naturally flexible. The teacher has probably done yoga for several years, and have practiced a lot. As with everything things take practice and that takes time.
As a general principle, yoga should not hurt. Although it can be strenuous and feel a lot. But not painful - there is a difference. Generally sharp pain should be avoided, while intense feelings can be endured, depending on the intensity of the feeling and your own judgement. You say you're doing weights, so I assume you're somewhat familiar with your body. As with weights, don't force yourself outside of your limits, and slowly with increasing the "load".