I've been down this road, but about 30+ years ago. It really helped when I spent some time teaching her to drive--I think it was having the steering wheel to hang on to while getting used to the "feel" of boating--especially in that short/freeboard challenged boat. An alternative might be a well placed "grab handle". As far as the kids are concerned, spending several low key days in a row at the lake playing in and around the boat and water will help acclimate them to the fun of boating. Once they get a taste, they'll love it.
We took the boat on vacation and after a little coaching I not only calmed my sons fears he is now totaly adictted to the thing. After thirty minutes behind the wheel the chip off the old block was jumping waves, sliding into turns, and flat out ripin down the river. Thats two votes to keep the boat, one more and it stays.