Ideally, you want to have a first offer ready to go prior to mediation, and you also want to have a fairly clear idea of what your bottom line is.
This is where your divorce attorney comes in—he or she will be able to guide you in deciding how to achieve your goals in mediation. In particular, you should prepare a budget (the court has a standard form for this) so that the child support discussion is anchored in real-world numbers. You should also prepare at least an outline of your desired outcome for parental decision-making rights and the parties schedule with the children.
This preparation is difficult to do without the help of an experienced family lawyer. Your lawyer’s experience is your guide to what is reasonable and appropriate under the circumstances of your case, and what is likely to happen at court if you do not reach an agreement.