Well I think that the only thing that you can do is try to be nice and be understanding of them. Since you have not seen or spoken to them in 5 years, then they may be resentful of you. They may not even want to talk to you. So you have to remember that.
If you could, you could start by talking to their father. Have them talk to the children first. Have him give the children good reports about you and how much you would like to see them. This way, if they become angry and say that they do not want to see you, they have their father there (someone they trust) to encourage them to see you. That is of course if he is willing to do that.
Also you can, start off by writing a letter. Tell them that you miss then, love them, and if need be apologize for not seeing them sooner. Ask there permission if they would like to see you. You do not want to force them, it could make them reject you. If they do not want to see your right away or if you are not ready right away. Continue to write and build that relationship. Send your gifts, cards, send pictures and ask for pictures of them. Try to create the bond through writing. Even if they do not right you back. Continue to send letters and things to show them that you care. Eventually they may write back.
As far as your son with special needs. You could ask his brother or father to try to read the letters to him. But write him a letter too, so that if he understands he can still know that you care about him. I sure someone in the home can read the letters and show him the pictures. Try to include him as much as possible. I do not know the extent of his special needs, but I think this would help.