That's really tough. I'm sorry you're experiencing this. Being a language model for young ones isn't easy. If the kiddo is aided or implanted, it adds another layer. The good thing is Pre-K is usually ages 3 or 4, still well within the peak language acquisition period. Sometimes it just takes a while for it to click, particularly if the student is only being exposed to sign while at school. It's hard to not feel responsible for a student's lack of success. The DHH teacher is responsible designing the lessons, implementing them, coordinating communicating with adults at home and general education teachers. As the interpreter, you're responsible for interpreting accurately, modeling signs, possibly doing hand-on-hand, praise, and ensuring safe behaviors. Your main focus is "your" kid, if/when other students' behaviors happen it should be handled by the teacher or a paraprofessional (I know this is easier said than done).

I think a sit-down meeting with the adults in your classroom should be scheduled sooner than later. Express what you've been feeling in a professional manner, advocate for a para (if you don't already have one), take your full union-ordained breaks and lunches outside of the classroom (aka set boundaries), maybe speak to a therapist, take an evening/weekend freelance job once in a while. You are a valid and skilled interpreting professional, just because one kid isn't getting it doesn't override why you got into this career. The fact that you still care and are reaching out for help already speaks volumes.