Is zakat due on gold jewelry that a deceased wife bequeathed to her children, and if it is due, how is the zakat for past years, which was not paid, calculated, given that the children were minors at the time of death?

Your wife's will regarding the gold cannot be executed except with the permission of the heirs, because there is no will for an heir. If the heirs permit it, or if you and her two children are the sole heirs, then zakat is obligatory on the gold designated for marriage if each child's share reaches the nisab and a full year has passed over it. Zakat for past years is obligatory because it is a debt owed by its owner. The two children are responsible for paying the zakat as they are the owners of the gold.