So if you two were together for 2 of the 3 years then you are liable for her social assistance debt for just one year. Beyond that it is her and the new guy vs the government.
I believe you said your son is with you so he isn't an issue in all this.
As for the 5, you have to pay whatever is ordered for the 10 years (or when they hit 25, whichever comes first).
I think that summed up the situation??? I don't know how the money for the kids is assessed but I think wherever possible you should pay the government back rather than pay them directly. You never know when they may double dip (well, I guess you do, always). And while they are somewhat safe when under 18, as soon as they hit 18 they can go on welfare too...
I think at this point I would look into a lawyer.