I have had sports doctors tell me that single handed backhands when combined with also using the dominant side for forehands, can harm the child's spine. They feel it causes a muscular imbalance that literally pulls the very pliable young spine to one side. So they recommend exercises to balance the muscles.
I think the 2 handed backhand first teaches them to rotate their shoulders as they have no choice. My feeling is to teach the 2hb first, then show them a 1 hander at an older age, just in case there is something to that muscular imbalance theory.