The translation su agrees in number with the noun which it modifies; their is translated by su, sus, depending on what follows: their father/mother su padre/madre; their books/magazines sus libros/revistas. For their used with parts of the body,
Here is a selection of useful words and phrases you will need in real-life situations while you're visiting Spain and other Spanish-speaking countries...
Churro is a typical Spanish food, consisting of a long thin cylinder of dough, deep-fried in olive oil and often dusted with sugar. Churros are usually eaten with a thick hot drinking chocolate, especially for breakfast.