J.M.
The same thing happened with my 18 month old a few months ago.
We put on the Hydrocortizone 1% twice a day and on top of that vaseline until the rash got better. After that twice a day we practicslly coat her with vaseline. Also limit the baths to twice a week or if it's really needed.
This is the only thing that works until the more humid weeks of summer come.