Update on this question:
At the meeting for the contract signing, I presented the client with the model release and explained its purpose. The subject of other uses came up spontaneously, and I mentioned that some of the shots of her daughter with her kite might make good figuratives if it was all right with her that I should use them that way. The client seemed pleased by the idea and asked to see the paintings when they were done, so it seems that particular hurdle is gotten over.
I'm very glad I asked about it here, since refining my approach and knowing the possible pitfalls was extremely useful. It's already come up again a couple of times, and I was able to come right back with a confident answer and reassurances.