I cannot even imagine the situation you have been faced with but the advice that has been given to you so far is great advice, especially to seek legal counsel about this.
A close friend of mine passed of cancer (after fighting it for 5 years) and she knew her prognosis was poor so she arranged for each of her two children to receive gifts and cards at milestones in their lives as well as for their birthdays. She bought these cards and gifts, wrapped them and gave them to a close relative that she trusted to take care of this for her and they have done so. She also did videos to each child as well as photo albums. She and her husband were also having serious troubles before she passed (he was cheating on her!) so he couldn't be trusted to keep her memory alive.
If necessary, down the road you could also look into Mommy's Light, an organization that takes care of the children left behind. Check it out...http://www.mommyslight.org.
Be strong and take care of yourself. Don't be afraid to stand up for what is rightfully yours and arrange to get your children back as soon as you are healthy. I would arrange visitation with them as well.