Why don't you try it at least?
Just to eliminate the possibility that the kids are being coerced, you may want to explain it to the kids and significance of doing the medicals whether or not they will follow you to Canada. Also explain what it means if they do not undergo the medical exam. Quote the wording about it from CIC's website(and provide URL link in case they want to confirm).
After that you can may be write a document like the one below (just an example):
I, [name of 17 year], born on [date of birth] to parents, [full name of mother] and [full name of father] on this __
19th__ day of July
2013 confirm that the above information and possible implications have been explained to me by [name and relationship or name and occupation of person explaining]. I confirm that I have read and understood the information above and my questions have been answered to my satisfaction. I therefore decline to undergo a Canadian Immigration medical Exam and I understand that in doing so, I can not be sponsored by my mother as a member of the family class at any point in the future. I confirm that this decision was made of my own free will and I was not coerced.
_____________________________ ___________________
[name and signature of 17 year old] Date
____________________________ ___________________
Witness 1 (their father) Date
__________________________________________ ___________________
Witness 2 (lawyer or notary or justice of the peace) Date
They should wait to sign it in the presence of a justice of the peace or lawyer or notary. Also check and confirm if a 17 year old can actually sign a legal document or may be his father needs to do sign on his behalf, being he is the legal guardian as well.
There could be a letter from their father confirming that he also understands the information, that is the children's decision and he supports it and will take care of them.
ujbaby said:
it is still hard to get this done cos in my country, things are quite hard to get done and my kids family wont give them any chance to do this. just don't know what to do. did ur partner applied with his dependant and later changed his mind?