There are a few places to eat beside the Colosseum and this place is on the southern side right opposite the entrance so you have a great view. They do lunchtime food such kebabs cooked on a grill, pizza's, sandwiches, omlettes etc.
There's a nice cafe in El Jem (whose name I've forgotten unfortunately) with excellent views of the northside of the amphitheatre. This side of the amphitheatre is completely open so you get a fine view of the interior, even from a distance.
The restaurants in El Jem are serving simply food in town center.But it seems there are better restaurant next the town hotel