When to use manual focus in a DSLR?

Ever since I got my prime lens AF-S 35mm f/18G I have got a couple of out of focus shots in portraits.

I have switched off my auto focus assist light when shooting low light photos of my kids. I find that the Auto focus is unable to focus at large apertures and does not snap the moment when I want it to.

In fact, if forced, it gives out of focus pictures. The best thing to do in low light with large apertures is to switch to manual focus.

An example of low light auto-focus

A better in-focus image in low light with manual focus