Fire ant bites

I took my littles outside to play. We’re usually good at keeping ant killer on the ant hills. If we didn’t our yard would be one big ant hill... somehow we missed one and my April baby stood in it and didn’t move or cry or anything. I was helping my oldest get on the swing and I look back and her poor foot is covered! I couldn’t even see skin! I’m watching for swelling and putting a cold compress on it. My step mom told me to apple cider vinegar on it. Should I give her Benadryl? Oatmeal bath? Anything else? I don’t want it to hurt or itch her...