There’s no question kindergarten is a good thing. More of a good thing can be a better thing: hence the national movement from half-day kindergarten to full-day kindergarten.

The Tacoma and Peninsula school districts have joined this trend with enthusiasm. While other districts have largely provided free all-day programs to their lowest-income students and schools with high poverty rates, Tacoma and Peninsula have decided to expand it to every school and student.

Good arguments can be made for this. Tacoma’s interim superintendent, Art Jarvis, says that running both half-day and all-day programs could require different curriculums, and that requiring families above the poverty level to pay tuition for all-day enrollment – as many districts do – can squeeze out the lower end of the middle class.

More fundamentally, he said, a full day of kindergarten simply prepares students better for the later grades.

That last point is at least somewhat controversial. Some studies find that the benefits of all-day kindergarten fade after a few years, though supporters say that this is because the curriculums of later grades aren’t designed to exploit the early gains.

And one 2006 study by the RAND Corp. – a very credible think tank – actually found that all-day kindergarten was linked to poorer fifth-grade math performance and greater emotional problems. The RAND researchers said their study wasn’t conclusive, however.

But let’s assume that all-day kindergarten is thoroughly beneficial for everyone. There’s still another question: Would the millions spent to provide it free of charge to students well above the poverty level be better spent getting at-risk children ready to enter kindergarten in the first place?

An enormous body of research indicates that early childhood education – pre-K and younger, provided by parents and professionals – is the single biggest factor in a child’s success in school. So if there’s enough money laying around to provide free all-day school for kindergartners from higher income brackets, why not steer that money instead toward younger children who are already falling behind by the age of 3 or 4?

This is a question for the Legislature, not the school districts. Free all-day kindergarten for everyone may be a perfectly sensible investment viewed from within the K-12 world. But the K-12 world is awash in money compared to the patchwork of underfunded efforts that constitute the state’s early learning system.

If the Legislature treated early learning as part of a single continuum of education stretching from birth past high school, it would be spending money differently. Maybe it would pay full freight for all-day kindergarten, maybe it wouldn’t. But it would certainly be paying more to get children to kindergarten ready to learn.