A lot depends on how fancy your SIL wants to get with her granddaughter's clothing. An older machine like a Singer 201, Pfaff 30, Necchi BU, or one of the vintage Japanese machines will probably cope better with the jeans than most of the modern machines.
However, your SIL should be able to find one of these older machines for under $100, which would leave her with money to buy a modern machine as well, if she would like to have some decorative stitches and automated button-holes etc.