I have been in a similar situation, so sympathize with you and your grandkids. What is different is that I can afford to travel to see my grandsons or bring them to visit me in the summer. It is a huge expense and I get no help from his parent/step-parent. For some reason. they think I have an unlimited supply of money. That's ok, what I spend on seeing my grands won't be in their inheritance!
Perhaps if you asked your grandson why he wants to live with you - no leading questions - he will tell you something that you can act on.
Other than that, I suggest that you call your daughter and grandson as frequently as possible.
I will pray for you and for them.
I agree with you - no one 'teases' with a pick-axe.
Just had another thought - if you have a computer or can get one and your daughter has one, you can get relatively inexpensive video cameras that 'attach' to the computers. Then you can call your grandson and daughter and see them, too. Of course, that only works if your daughter co-operates.
I like the idea of a code word to indicate something is drastically wrong for your grandchild, but, he may not be old enough to understand when to use it.