Would I be sinful if I did not take back my children, whom their father took from me, especially since he abuses stimulants and blackmails me with them, and I intend to marry to protect my chastity?

You have a greater right to the custody and care of the children after divorce, and their maintenance is obligatory upon their father, and he is not permitted to abandon it. If you relinquish your right to custody, or if your divorce was a khul' (a divorce at the wife's instance) in exchange for it, then that is permissible for you. You do not sin by leaving the two children with their father if you declare your inability to provide for their expenses and care. The father is not permitted to prevent you from seeing them or maintaining ties with them.