The bilingual student can be a great asset for you in class. You can define an activity or game and then split the class in two and have the bilingual speaker take charge of the other half.
It is quite normal otherwise for the bilingual child to be bored stiff, I mean wouldn't you be! If you cannot give her the task of helping the others out, so she can learn other skills and feel like she is contributing, then I should let her do her homework for other subjects, anything other than have her totally waste her time.
I would not have any qualms about speaking Spanish in class in order to explain something such as how to play a game. If you speak the native language that is an asset. Give some brief instructions in Spanish then play the game or do the activity in English so that most of the time the lesson will be in English, but the use of Spanish will just help you along and keep the pace going.
To keep the children motivated make the lessons fun. Songs, games and theatre are the most fun in my view.
For game ideas please see:
(for 6 to 12 year olds)
(for 3 to 5 year olds)
On both of those pages you can sign up to receive free games and tips that will help your children enjoy class more, and you too!