Since the children are non accompanying, you don't have to pay application fees for them. However, they have to get a medical exam and must be included in IMM5406 form.
According to the Sponsorship of a spouse, common-law partner, conjugal partner or dependent child living outside Canada,
"All your family members, whether accompanying you or not, must be declared on your application and be examined. If family members are not examined, it is generally not possible to sponsor them at a later date. This includes children in the custody of a former spouse or common-law partner.
In addition, failure to declare family members on your application and have them examined goes against your duty to provide truthful and accurate information, and may cause you to be found inadmissible to Canada."