hello thanks for your question,
It is very likely that you are dealing with either Seborrheic dermatitis as you suspect or a bactorial skin infection which happens often when the lick the open sores created by scratching.
If you really want to try over the counter you can try a monistat cream used to treat yeast infections, apply 2x daily.however if after 5 days you see no change I would recommand having her seen by a vet to rule out a skin infection and or have your baby placed on antibiotics. If you do notice a change I would continue to use for 10 days
I would also recommand cleaning the areas daily with an antibactorial soap.
You may also want to pick up a halo collar so she can not lick while you are trying to clear this up
You can swab her ears rub on a slide ask your dad if he has gram stain which I am sure he does stain the slide then you want to look for binding yeast and bacteria.you can also do this on area of effected skin. For the skin you can use a piece of clear scotch tape press the tape on the skin then put the tape on a slide and stain as suggested, check for the same thing
Head and shoulders for people works real well for the dermatitis an oatmeal bath works well too if he appears itchy.