Add a filtered state to your child node, then define your equation.
For example you have a "continuous" parent A and a discrete parent B and a child C:
We suppose that C is not possible (filtered state *) when A is less than 0 or B is not equals to "OK",
you can define the equation (a part of the equation for the impossible combinations) in C as:
if(?A?<0 | ?B?!="OK";"*";[...result of the valid combinations...])
where * is the filtered state of C.