This link provides the list of indications that are granted an automatic waiver from pediatric studies/trials. Note that oral mucositis is not on the list, so IPIX had to request a review by FDA. While it is likely that it was a relatively easy waiver to get for OM, it still reduces the potential trial size and I imagine an interested party (and IPIX) would want to make sure it was granted before moving forward.

https://www.fda.gov/media/101440/download