This is an event for preschool children.
There will be an information session for parents of preschool children.
Also means 'preschool' or 'before entering school', but 未就学 emphasizes the state of not yet being enrolled.
Compound of 未 (mi, 'not yet') + 就学 (shūgaku, 'entering school'). The exact historical derivation is straightforward; the term is modern and primarily used in administrative and educational contexts.