There are very few reasons a dependent would ever be rejected under a family class application. Some of those reasons are:
- if child was not declared/didn't do medical in parent's own PR application
- if child fails medical (very rare unless they had some contagious/dangerous illness)
- if child didn't qualify as a dependent (so if they were over 18)
- if CIC did not believe the child was actually biological child of parents, or they failed a requested DNA test
- in cases of adoption, if CIC feels it was an adoption of convenience
Other than these main issues, there is no reason CIC would otherwise deny a child's PR application.