Thank you for your query.
Individuals do get such lesions on the hands and nothing needs to be done unless they are increasing in size. As you noted they subside on their own. I suggest that you wait and watch.
You asked what is this and what could it be. Toddlers do play around and also with mud, etc. They can have small abrasions, injury
or insect bites
which may go unnoticed and these could be the body's reaction to it.
I hope I have answered your query to your satisfaction.