Is this somehow illegal about this?

You need to reestablish your relationship with your children. Since so much time has ped, it would be wise to use a children's therapist or counselor to act as an intermediary. As much as you say or think the children are missing you or need you, it is going to be extremely traumatic for them to have you back in their lives after such a long time especially the youngest whom you have never met. Forget the blame issue since it is and always has been your responsibility to ert your parental rights. It is obviously also the mother's duty to make sure there is frequent and regular contact between the father and the children including notification of address changes and other important events The courts would end up blaming both of you but they would never agree to let you back in the children's lives without some sort of professional intervention subject to an ongoing plan for regular and frequent contacts. If you end up not seeing the kids except for once or twice a year, it would probably be better to cease all contact.