Is marrying the widow of a martyr who has children better than marrying a virgin, and is there any Prophetic Hadith reported regarding the virtue of that?

One cannot definitively attest to a specific individual being a martyr, even if we hope for that to be the case. Rather, we say, "It is hoped that he is among the martyrs," for Allah knows best what is in intentions. We do not know of a specific virtue for one who marries the wife of a martyr or takes care of his children, except that the guardian of an orphan in general, including the children of a martyr, has a great reward. The reward for caring for the children of a martyr may even be greater due to the martyr's virtue. If one's soul inclines towards a virgin, that is natural. However, marrying a non-virgin who is a mother of orphans, while being capable of fulfilling her needs and those of her orphans, contains blessings and provides chastity for her and her orphans. If one's soul inclines towards a virgin, it is not advisable to force it towards a non-virgin. It is recommended to perform the Istikhara prayer and to be deliberate in the matter.