What chemical can cause a burn on baby skin like this?

I have a photo I have taken of what we suspect is a chemical burn on my 21 month old little girl. Please if you have experience with burns, I would like to share the photos so you can see the damage. We don't leave household cleaners out or anywhere our kids can get a hold of. She was at the babysitter and the sitter called to tell me she thought she was stung by a bee a few times because the baby was outside playing and suddenly started screaming and crying and saying "ouchy, boo boo" and pointing to her arm. The sitter said she put neosporin + pain relief on the affected area. By the time my husband picked the kids up and brought them home, he gave them a bath and when I got home, I saw the area and it had bubbled up on the one larger area and become pus filled and the skin started to peel off and was oozing. Took her to the ER where they suggested a chemical burn. The baby had on two layers that day with long sleeves so, I'm wondering what could have gone through both layers without damaging the clothing, but enough to affect her skin??? This is really odd and it's been 5 days with no real improvement. The docs put her on oral antibiotics in case it is a bacterial infection, as well as topical antibiotic ointment. I have been cleaning and redressing the areas every morning and evening and still not improvement. The areas are still oozing and are purple with skin peeling off. Has anyone see anything like this before? Could a plant outside have caused this? We live in the suburbs in Ohio, so I'm not sure what species of plants around here could do this. Docs aren't proving to be much help.