No. The official family terms describe a spouse and unmarried children under twenty-one, each with a separate fee and gift. Grandchildren are not included at any age.
Two generations down is outside every derivative rule
Derivative concepts across the immigration system extend to a spouse and unmarried children under twenty-one, and the official programme description follows the same pattern with a separate processing fee and gift for each accompanying family member. Grandchildren have no derivative claim through a grandparent under this framework, and neither do adult children. Where the couple's motivation is to be near grandchildren, the practical question is what status the grandchildren's own parents hold, because the family's route often runs through them rather than through the retirees. Where the adult child is already a United States citizen, a family petition for parents exists and is worth pricing against a gift, since it involves no payment of that kind. Get the current official family terms in writing before assuming any of this.
Hypothetical example: a blended family includes a spouse, a seventeen-year-old stepchild and a twenty-two-year-old child who is financially dependent. A first review produces marriage and birth records, custody evidence where relevant, and an age chart for every person proposed for inclusion. Those records settle who can be included under the published family terms; financial dependence does not turn an adult child into a qualifying derivative. The family must decide whether to include eligible relatives at the initial stage or plan a separate route for those who cannot qualify. Assumptions made after payment may be costly because each included family member has separate programme charges.