Microsoft Word has a grammar check. It also lets you know the rule(s) that is/are being broken. If you want to work on your own grammar, to help your child, then I recommend MSWord. You can type out something you are having a problem with, then look it up, all in one program.
IMO, showing your child that you are not perfect, but willing to work towards perfection, is just as good as knowing everything. My kids know how to use reference books because I use them all the time. I may not have all the answers, or the best spelling or grammar, but I am willing to look it up.