Unless the offender is currently on a form of community control (i.e. probation or parole) and the conditions of that control prohibits it, the offender has the same rights as any other citizen. That goes for the woman. If her custody decree states that she cannot bring the children around sex offenders, then she's not doing anything illegal. If the children aren't being harmed, you should do nothing. The mother should be able to parent her children the way she deems appropriate (even if we disagree with her choices).
This information is intended as general information only. This communication does not establish an attorney-client relationship between me and the asker.