Not a bad way to start. Keep in mind though, (and point out to her), that the problem is you pulling the pedal out from under her feet. So, in order for toe clips and straps to stop that happening, eventually she'll you have to learn to use them properly. That means reaching down and cinching them up tight around your shoe, and having to reach down and release them when it's time to dismount. So obviously, they are harder/take longer to get into and out of than clipless. That's why clipless were invented and adopted by everyone.

Tandems, incidentally, are also an excellent training ground for clipless stokers. Once they get confident with your ability to hold the tandem during starts and stops, they know they'll never needs to clip in or unclip in a hurry. So they're much more amenable to trying clipless. Something to consider for the future perhaps.