I believe possibly that the mother was focused on exclusively to perhaps appeal to single parent mothers or/and women who have children where the father is not in the organization. I believe the intent was to make the video more appealing to them to see the mother alone dealing with a child. You can have quite a number of single parent children with just a mother or the father being a "non-believer". I believe it is an indirect appeal to them on a more subconscious level without pushing it too much for it still to work for the "traditional" family arrangement as well.
I could be giving them too much credit, but I think it plays a part.
I think it also, whether intended or not, makes the message of the video go down all the easier for those mothers who are single parents. They are happy to have something from "Jehovah's Organization" showing them in charge and dealing with the child, so they best follow to the letter the Society's message to them. It works to both make them feel special by being 'helped' by the Society, while making them also feel singled out and guilty to ensure they are bringing up their child in the "mental regulating of Jehovah"...
Yes, I almost threw-up even just typing that last statement.