Legally you cannot just agree with your spouse on an amount of child support. In Canada our law says that child support is the right of the child and therefore it cannot be given away by a parent. Child support must be paid in the amount stated by the Child Support Guidelines.
If a lessor amount is being paid then the support payor must prove that they are supporting the child in an alternative way which is acceptable to the court and both parties (i.e. paying private school tuition or a residency fee).
Practically, there is very little oversight over what happens between separated couples “behind closed doors”. If nobody complains about an improper amount of child support being paid then most likely nobody will do anything about it. However keep in mind that the court will not grant separated couples a divorce unless both parties swear to the court that there are proper child support arrangements in place. It is considered “perjury” which is a crime to lie to the court knowingly.