Japan is pretty big on chibi type characters and that's all Lala's really are. They may have the appearance of a child but it doesn't mean they are one. In fact we don't know how old most are.
Here's the definition from the FFXIV website.
"The Lalafell are a folk both rotund and diminutive. Small by any race's standards and possessed of a childlike countenance, it proves difficult for non-Lalafell to gauge an individual's age with any degree of accuracy."