Since you still have a godparent in good standing who is active in the lives of your children, then perhaps you might want to consider turning your children’s attention to what they owe to the absentee godparent, rather than concerning yourself with what the absentee godparent owes your children.
The godparent relationship is not a one-way street, in which godparents are expected to provide emotional support, spiritual mentorship, and tangible goodies (such as birthday cards) while the children are but receptacles of bounty and negligent godparents who fail to fork over the goods are stripped of their godparenthood and replaced. Godparents do have responsibilities to their godchildren, and deliberate negligence and absenteeism cannot be excused, but godchildren are expected to participate in the relationship by offering their love, prayers, and sacrifices for the sake of their godparents.
In this case, you have a godfather who has gone through a traumatic life experience. For whatever reason, which may include not feeling comfortable intruding upon a relationship in which his wife is still an active participant, he has not been able to maintain contact with his godchildren. Rather than seek to cause him more trouble and loss by stripping him of godparenthood (which is not possible anyway, as the Church will not allow it), perhaps you should encourage your children to offer their prayers and sacrifices for their godfather. Who knows? Maybe God can use the merits of your children’s spiritual offerings to help this man get his life back in order.