I have never read their books, but have heard, from good and devout Traditional Catholic father that the parenting methods taught in their books have very practical applications for raising children. They note that the authors are Prots, so one must take their writings with a grain of salt, but that their methods of raising good, respectful, well behaved children are in practical application, quite Catholic.
And considering the person who told me this has done a very commendable job raising children that are respectful, well behaved/mannered, polite, and appear to love and practice their Catholic faith, leads me to believe there would be nothing harmful in reading through their content.
Again, fwiw, I haven't read them, so I can offer no detailed insights, just relaying what I've been told from someone that I feel is a reliable source. Hope that helps.