There is no single answer to this question. It depends on your relationship with your parents or in-laws, and your child’s relationship with them.
If grandparents don’t have contact with your beloved, chemically-dependent child, why tell them about the pain and struggle? If grandparents are fragile and sick, don’t add to their worries. In short, if there is no reason for them to know, then there is no reason for them to know. Alternatively, if they are close to your child, they probably already have concerns and may actually be relieved to learn you’re taken steps to get help. And they may be able to offer financial resources to help with rehab.
Possible “land mine” ahead: many of “the Greatest Generation” are in the dark about the biological origins of chemical dependency and may consider this a character or personality defect. Be ready to explain to them that your child has a brain disease and has become chemically-dependent upon drugs or alcohol. Help them understand that there is great hope for sobriety, and that you are taking steps as a family to get help.
It’s critical that grandparents understand you need to be united as a family. That means Grandpa cannot give the addict money, and Grandpa cannot make her home a safe place for the alcoholic to crash. You all need to be united in the conviction that professional assistance is necessary to help your child get healthy again. You need to circle the wagons around your child.
This is not a time for blame or guilt. (Is there ever a time for blame and guilt??) No one made your child an alcoholic/addict, any more than anyone could make him or her diabetic or allergic. No one has the power to make another person chemically-dependent. And no one but the addict or alcoholic has the power to reclaim their sobriety. Long-term recovery is within reach: 23 million Americans have already claimed theirs. If you enlist Grandma and Grandpa in a loving and informed way, your child will have a healthy network to help them claim their own recovery.