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"Applying for Canadian Citizenship:

A parent, adoptive parent or legal guardian must apply for children who are under 18 year old. Parents must apply for citizenship for themselves and their children at the same time." http://www.welcomebc.ca/Immigrate/Already-in-B-C/Becoming-a-PR-or-Citizen/Become-a-Canadian-Citizen.aspx

From what I've read, there is nothing stopping the father from applying for the children's Canadian citizenship.Apparently, all he needs to do is apply at the same time for naturalization.

Also, I might be paranoid, but I think the father is trying to do something else by gaining Canadian citizenship.

"Canada, per its Supreme Court rulings, will not recognize a US custody decree against a Canadian parent, regardless of any other considerations. Including parental kidnapping." - This quote talks about a US custody decree, but I'm not sure if this is the case with UK also.
It's possible that he wants to take over the custody from the mother.

I suggest you go see a lawyer. If you don't trust your "soon to be ex-husband", make sure that your children cannot leave the UK. Revoke their passports, get a court order restricting your children to leave the country, etc.
 
I still think that the OP's ex husband will not be successful in getting his children canadian citizenship on account that they are not living in Canada at time of application on top of that the children have not met their PR RO.
 
Consent for minor children

Hi I just want to ask, do I still need to get consent from my ex common law partner if he already abondoned us and still have the guts not ti sign on the consent form. What will i do? And one more thing, he left us when i was pregnant with my youngest and didnt sign on her birth certificate, thats why it appears on her birth certificate that she didnt have a father.