I think some here are over-thinking the five-year-old stuff. Staying on the bike is only 50% of the battle. She will also need starting/stopping, accurate steering, consistent pedal rotation, hand braking, endurance, and riding with others. Shifting does not sound like a priority.
My suggestion is to put the SO on the regular bike with pedal, helmet, and everything in a large empty parking lot and let her have at it. She is obviously highly motivated. Go ahead and drop the seat a bit.
Remember that beginners often steer to avoid small cracks.