My catechists are uncomfortable teaching this. What can I do?
It is understandable that teaching something like this can cause concern. No one wants to think about something this horrible, but all adults are responsible for protecting children. Be sure to provide training for the catechists and review the lessons with the catechists prior to the lesson. If the catechist is still not comfortable teaching this, the pastor, DRE, or another catechist should be sought to teach this particular lesson.
My catechists are concerned they will not know what to do if a child discloses abuse to them
That is a common fear and training in this area is very effective. We do not investigate, we report. Catechists and/or the Director of Religious Education should know how to report suspected child abuse in their city or county. Adults need to remember this is about protecting children. All they need to do is hear what the child is telling them and do what is necessary to protect the child.