CIC does not render everybody inadmissible who fail to include their spouse/child. I don't even recall how many people have come here and said nothing happened to them except they were not allowed to sponsor their spouse. I remember one who said that he himself lost his PR as a result of that.
If you look at OP2, the manual for family class, it does state that a dependent child who was not declared can not be sponsored as a member of the family class but could however gain PR as a family member on a new application, see http://www.cic.gc.ca/english/resources/manuals/op/op02-eng.pdf on page 16.
Further, it says that if you commit misrepresentation that would have been material to decision making on your application, you would be banned for 2 years. Not including your child is not such misrepresentation because it would not have affected your application at all. If you had been married and failed to include a spouse, that would have affected your application because if you had been married, you could not have been a dependent child on your mothers application.
You should talk to a lawyer to make sure you do everything right from now on. If you want to be completely honest with immigration, you would report yourself to them and see what happens. If they take your PR and ban you for 2 years, that's another thing but then you can be with your gf and child in the Philippines and work on your PR application for later.
