I don’t think you need to worry about it. Most teachers are pretty professional about it. As long as you are professional and courteous at what you say, you’re fine. You could just say, my kid would like to try other things and we are taking a break… It’s the easiest thing to say, and everyone knows what that really means when someone says that.
Besides, 2.5 years is not really that long enough such that a teacher would be butt hurt, especially when a student is only age 7. Many of the young kids aren’t really going to be interested moving forward anyway, so teachers already expect a turnover at this young of an age.
Also, the relationship between the student and teacher, in many cases is two way. My kid’s violin teacher has emphasized many times that he will let students go at age 12 if they aren’t performing at a certain level by then, and he doesn’t take any new students older than 12.
So again, it’s unlikely your teacher would be offended or upset about it, because even teachers know that not every student and teacher match up is always optimal. And teachers are also looking for students that match well too for the long term.