noun
full-day child (aged 0-2, at a certified early childhood center)
Administrative term used in Japan's certified centers for early childhood education and care (認定こども園). Refers to a child aged 0–2 who attends for the full day, as opposed to shorter programs.
この認定こども園では、３号園児の定員は１５名です。
At this certified early childhood center, the capacity for full-day children aged 0–2 is 15.
３号園児の保育料は、世帯収入に応じて決まります。
The childcare fee for full-day children aged 0–2 is determined based on household income.
Compound of ３号 (number 3 category) and 園児 (kindergarten/nursery child). The numbering system is administrative, not etymologically complex.