When you mentioned seat, I thought.of a folding seat at CT that may work for.you. On the CT website, search for Escort stadium seat pad. Part # 76-1599-0.

This, is a thin folding seat that will not increase the center of gravity and will help keep the daughter more upright.
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.

John Lubbock