Long time no hear....
Just a quick one...
Does the father maintain contact,
Do you have contact details
Most passport / citizenship matters relating to children require the signature of both parents unless sole guardianship has been granted.
However most applications recognize that this is not always possible and allow for an informed decision to be made without it.
You can do a number of things....first I would look at the application form notes, if it says that if the other signatory cannot be obtained explain why, you have the opening. If you or the children have has no contact then I am sure they would consider the application on its merits, particularly if good reasons for the application are given.
If you are in contact, then I suggest you send him a letter with a set of application forms, in the letter express the wish to get them their foreign citizenship, detail why you wish to do so, why you feel it is in their interest to do and request him to either sign the forms or to supply a written reason why he refuses to. Send itregistered and see what happens. If he is being unreasonable or refuses to reply, you will then have evidence to that effect and that can then be put before a judge here, who can then order the authorities to issue.
The bottom line here is that everything must be done to better the lot of the kids, leave any benefit to yourself out of it and ensure that anything you do and say is to the benefit of the children.
You need any more help, send me an e-mail or PM and we can take it further.