That left-handed forehand can certainly help with "activating" that left side for a righty. Compared with a pure forehand move though, I like to stress the extension of the left arm (for a righty) through contact to better "release" the racquet and find an optimum swing radius. This extension is a little different from a left-handed forehand.
I also like to have the kids put down their racquets and just throw a ball underhand with their left hand. The idea is to get them more hip to the feel of the step-and-turn move that drives the stroke. That throw is easier when there's a deliberate drive onto the right foot along with a shoulder turn. When the kids can recognize that extra energy in their throw, they can often translate that move into their strokes. Then they can be more likely to use those bigger muscles for the two-hander instead of too much arm/wrist action.