If they can sit quietly through a 90 minute show, then yes. However, I have been to shows with three year olds behind me kicking the seat the entire time or talking through the whole show. I find that disrespectful to the other people there (esp. since it isn't cheap). But, all children are different and you know your children better than anyone. I would definately use your discresion.
Another option is to take the oldest child as a special treat. There are some children's activities or even babysitters that will take your kids to the park (even though you have to pay their admission) for the youngest. It all depends... I have only heard good things about the babysitting.