Please keep in mind that this was one time. You witnessed her say something loud one time. This may or may not be the norm. We all have bad days, and it's not fair to expect daycare/preschool/school employees to never have bad days.
Also, it is possible that she was trying to be silly. At least, if you just read your quote and don't really know what tone of voice she used, it's very possible to imagine her being silly and trying to get the boy to smile and try again.
All that being said ...
My youngest had a teacher (I'll call her Mary) who did yell and say things to the kids in anger. This was in the 4' & 5's room (the classroom just before "school age"). Mary was a very nice woman, but she did get impatient with the kids. I could see myself feeling just like she did ... that's why I work with college students and not in a daycare! I probably should have said something to the director, but I didn't. One day I brought my son, and the director was subbing until the other teacher arrived (guess she was coming in a little late that day). You would think Mary would be a little more aware of her behavior with the director there. Nope! So I left the room with a little smile on my face knowing that there was no longer a need for me to talk to the director. Either Mary's behavior would improve, or Mary would no longer be in that room.
Mary's behavior did improve, but she was only at that job a few more weeks.
Honestly, I still think Mary is a wonderful person. She's just not meant to be working in a daycare center.
If you really think it's a problem, talk to the director. Just let him/her know what you saw and what your concerns are. Then let him/her handle it. It might have been a one time thing, it might be something that can be corrected, it could be a real problem. But let the director handle it.