Translation:Her baby often puts its fingers into its mouth.
It is very common to use "la" instead, just like it is in many European languages.
Mia buŝo = la buŝo
In my native language Norwegian it would be the same:
Hun stikker fingrene i munnen (She puts the fingers in the mouth)
It is only if it needs to be specified for some reason that you would use a pronoun here instead of the definite article, same with Esperanto.