Sexual assault on a minor

What is the name of your state? Wisconsin

My daughter recently informed me that my ex-husband, her stepfather, sexually molested her when she was 8 (she's 21 now). Are there any time limitations in reporting this?

See http://www.wcasa.org/pages/sol.html

Under my quick reading of that, it looks like the statute of limitations has run on prosecuting him because the offenses occurred before 1994.

I'm very concerned because this guy has joint custody of our 13-year-old son and has him three nights a week. Could this be brought up in court? I really want him to seek counseling for this, as I believe he may have done this to his nieces also.

Counseling is known to be very ineffective in treating child molesters. There is nothing you can do for him. The real issue is whether your son is safe. You have to have some kind of evidence, for example the testimony of one of the girls, to bring this up in any proceeding against him for child custody.