Typically you have to go with relatively low impact journals. Specifically with regards to pediatrics and case reports, I know Pediatric Critical Care Medicine takes case reports. Many surgical journals and device journals also take case reports.
Most medical school libraries have access to "Journal Citation Reports". Using this tool, I would look for low impact factor journals (~2 or less) within Pediatrics or whatever other category you think the case could apply to and see if those journals take case reports. You may also want to look at "Letters to the Editor" as these are sometimes case reports with reviews in some journals. You should also talk to whoever the corresponding author of the case report will be and ask for their opinion.
