Beginning students appreciate strings that are easy to play and easy to keep in tune. It also helps to select strings that are relatively forgiving - strings that produce a beautiful tone even when the player's form is less than perfect. They do not need soloist strings with great brilliance and projection. On the contrary, brilliant strings will accentuate any squeaks and squawks. Consider strings with a synthetic core, low tension, and a warm/dark tone.
It's worth noting that many viola rental programs provide students with inexpensive, plain steel strings. However, this is due to the low cost and long life of these strings, not because they are particularly appropriate for young players.