Not sure but it doesn't sound like you are trying to redact, rather it sounds like you are correcting your kids work?
Also, it sounds like you might be using Adobe Reader which really cannot do editing (or redacting for that matter). If you really want to edit your kids work after he's made his/her entry into the homework, you'll need either Acrobat DC or Acrobat PRO DC (the Mac only can get Acrobat Pro DC).
Forgive me for interjecting here and IF I'm wrong please ignore this but if your kid is making mistakes, they are his/her to make. One learns by making mistakes and correctig them, not having someone else correct them so that mistakes are not pointed out. Let the kids hand in their homework, not yours. And again, if I'm wrong here, please please ignore this.