scared to let my ex, kid dad know

I am terrified to let my kids dad know I have cancer. He basically hates me, and already once tried to get full custody when we broke up few years ago. Of c course he didn't, he had no tell argument, and has not even been able to keep to his days of visitation. But he is the kind of person, who would probably take me to court, to get custody so he won't have to pay child support, if he saw an opportunity. So this means I really can't talk to my kids about what's going on with me, their 4yrs and 6yrs. And I don't feel comfortable asking them to keep something from their dad. Has anyone been in a similar situation?