Inanna do you have a private agreement on what he should contribute to for the children? If he isn't paying his share would you consider having him assessed for what CS says he should pay? That might give you an idea of what you and your children are entitled to. Perhaps your not getting the amount of support financially that your entitled to. The amount my ex has been calculated to pay (and still doesn't even no the amount is increasing every month he doesn't pay) does not come close to covering DD's private schooling.
I guess if you have a private agreement ideally you want to keep it that way and agree to something that seems fair and reasonable for all parties involved. My ex currently has to pay $40 a month (it use to be $400) but my friend who has one DD ex's pays her $1000 a month. For both of us we have elected for CS to collect payments due to the other parties not following through with past arrangements.