Children need to be kept busy with whatever it is they are doing. The busier a child is, the less likely that child is to get into trouble. Young children are also very curious. Something catches their eye, they want to see it, touch it, and explore it. Kindergarten teachers have the dubious distinction of creating activities to keep these young children busy.
For parents and professionals, students being in all day kindergarten alleviates any day care issues. It also allows the students to be more socially developed, having spent a full day in kindergarten. Children appear to be more socially ready for first grade and all subsequent grades after spending a full day in kindergarten.
Since public education is funded by the state,s as well as by local taxpayers through property taxes, there is a cost involved to having all day kindergarten. Implementing full day programs requires more money from local and federal budgets. There is also the matter of whether a state will or will not require all day kindergarten, per that state's policy. The could mean inconsistency per the student as the student advances through his or her scholastic career compared to other students.
A focus of student lasts only so long before he or she become bored. Young students may be prone to burn-out from all-day school. Children will get restless and tired, causing them to lose interest and it could also pose a problem for other students. Youth at this age may not be able to deal with the long hours of instruction.