I suspect most of us who have lost someone who mattered to us have had the experience - hearing their voices, seeing them at times, maybe even sensing their presense or their touch. Just recently, I thought I felt my mother's cheek in my hand when I awoke in the middle of the night and began to talk to her. Was she there? I don't know. But it gave me great comfort and a sense of connection across the veil to the other side, and warmed my heart.
Now when we are dealing with children who are trying to make sense of a death, seeing Grandma may give them comfort, so I would not take that away from them. Depending on the age of the child and their development, a skillful parent might be able to use the imagination of their child to work with them to understand the loss. But that is up to individual parents to decide.
For adults, I find that these images and conversations can also be very helpful. As a matter of fact, in my classes on Recovery and Reconciliation, I encourage people to speak to the deceased. This helps say things that needed to be said, speak the unspoken truths that death often robs us of. Grief is often wrapped up in things left unsaid and tasks left undone. I find nothing "nuts" about having those conversations now.
The relationship with one who has departed does not end with death - it changes its' shape and character. How that shape looks depends very much on the nature of the history you have with that person. One can say Good-Bye to beloved family and friends, one can speak things aloud that needed to be aired, without fear of being contradicted. And I find it very powerful and transformational to be able to use this tool to work through the grieving process.
One caveat about conversations with those with whom we have had challenging or toxic relationships. It can also be empowering to speak to them, but if there was an abusive or other destructive history, seek out the support of a therapist who is trained to assist you specifically in healing from that damage, in addition to working with a grief coach. Better to have more support and tools at your disposal.
No one should ever be called Crazy when they express a true emotion. Especially when one is vulnerable, having your feelings validated and honored is absolutely key to the healing journey. If someone shares their feelings or experiences with you, even if you may not relate or understand, just listen. It takes great courage to open up to another person on such fragile subjects and a loving ear can be a balm to a wounded soul.