Trying to remember the name of a book

Looking for the title of a book I read in grade school. I don't remember much, except for some key elements of the plot, and even those may be skewed from years of new knowledge messing with the old.
Okay so the book is about a young girl who goes to see her Grandmother, and some how aquires a science kit. She makes a formula that is supposed to do something (I dont remember what though) to her grandmothers hair. But when she washes her grandma's hair with it, it causes her grandmother to begin aging backwards. I think they lived by the beach, and for some reason I remember one part where the girl is pulling her grandmother along in a wagon, because her grandmother is now to young to walk...wasn't a very long book, and I dont remember much else, but it bothers me not to remember the name...

please help~

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