Taking time off work to be a stay-at-home parent falls under family-related reasons for an employment gap.  The best way to explain such an employment gap is to write down the exact period you were off, titling it as a “planned career break.” Underneath, you can include one or two sentences on why you took it (e.g. took three years off work to care for my growing children, who are now in elementary school).  On top of that, you can also mention any personal projects or other work-related experiences you had over that period. For example, if you took some online classes or started a personal project, that’s a great way to inform recruiters that you weren’t completely idle during your employment gap.