I don't know what a social study is. Is that like a custody evaluation?
Rosebud, it sounds like your x and his allies could be engaging in a parental alienation campaign (consciously or unconsciously). Were the children previously very open and agreeable with you? I would start reading up on how to counter the effects of this. Some titles that come to mind are "Toxic Divorce" and "Divorce Poison". "Hold onto Your Kids" (and other titles that address attachment/communication) may be helpful too.
I agree with Rubelin that honesty is best, and for the conversation to be at their developmental level. Sharing just the verifiable facts (without the interpretations so much) is still being respectful. But since you have a court order to not discuss divorce I would be very careful and probably not give any sort of response that the kids could then turn around and tell a custody evaluator or your ex. You may want to be able to prove that x is discussing the divorce with the kids....and it will make him look worse to a custody evaluator or judge. Be sure to document everything that the children tell you (note the time, date, context and what you said to them). You could ask your lawyer what kind of response would be okay in this situation. Or perhaps you could give a vague answer such as "When people divorce, one parent is usually required to pay child support to help the children to have a similar standard of living at both homes. The idea is to make it fair so that one parent doesn't get 5 disneyland trips while the other has none" (or whatever types of things your kids can relate to). Your father and I are in the process of working something out that will be fair. We both love you and will do our best to ensure your needs are met at both homes"
Other mamas might have better suggestions. I do, however, feel that in the long run, it is important to correct any misperceptions they have about the facts (within whatever is acceptable re. the court order), but also not say anything to cause them anxiety or to feel responsible. And to avoid being emotional about it. They deserve to have all the facts on the table so that they can then come up with their own perceptions about the situation. Otherwise, if all they hear is misinformation - they may start to believe it and start buying into x's brainwashing. Do you have access to counsellors for the children?
If it's clear that x is discussing the divorce (in spite of the order) you can remind him in subtle ways about the order. If he continues to violate it, then you might try to bring it up to the people who could influence custody decisions. And if nothing gets done about the court violation, then you may want to consider not having that order at all (since it's not helping him behave, and it would then allow you to speak truthfully to the kids without recourse).