Do you think you could talk to the parent that witnessed the situation, maybe say "I feel like I need to explain myself from earlier....."? This happened to me a few weeks ago but it was the Dad of the child I raised my voice to that walked in. I had already asked this toddler 3 times to pick up 2 toys and finally looked over to see them still sitting there... I said in a stern voice " PICK UP THOSE TWO TOYS NOW SALLY"!! The looked up to the Dad standing there, he said "has she cleaned up everything you asked? " then took her right out without saying anything else. I talked to the mom that night to explain the situation and hoped they didn't think I was always that stern but sometimes I had to be? She totally understood.