My only son passed away in the blessed month of Ramadan. I do not have a husband or anyone to support me. I would like the legal opinion for the following:
1- Is it recommended for me to relinquish all or part of my legal share in my son's estate? Should such a relinquishment — whatever its percentage — be in favor of my son's wife or also in favor of my young grandchildren? What is the recommended percentage?
2- Since I do not have any children to inherit me, is it possible for me to leave a will in favor of my three young grandchildren (2 girls and 1 boy) to be implemented after my death? What is the permissible percentage that I may leave of what I own?
3- Apart from the will, is it possible for me at present to buy — without any limit to the amount — a property, shares or securities for my young grandchildren?

1- There is nothing in Islamic Law that exhorts grandparents to relinquish their share in a son's estate in favor of their grandchildren. However Islamic Law urges one to leave his heirs — especially if they are young, needy, or have special needs — prosperous and in no need of others. The Prophetsaid, "To leave your heirs wealthy is better than to leave them in need and begging from others" [Recorded by Bukhari and Muslim through Sa'd ibn Abu Waqqas (may Allah be pleased with him)]. We advise you to relinquish your share in favor of those of your grandchildren who may want in the future.
There is no prescribed percentage but the matter revolves around what you see is in the interest.

2- You may bequeath one third of the estate to your grandchildren.
3- Yes, you may do this.