Again: once your kid turns 5 (4, if you're rich and you send your kid to private all-day preschool), you're generally sending them somewhere where one adult will watch as many as 30 kids concurrently, all the while educating them to the point where they can creditably pass standardized tests. That's a harder job than just making sure kids are happy and engaged, and yet we pay far less for it than we do for child care.
Child care is a huge part of why people get crappier jobs than they might. You can't go back to school if you have no savings and need to pay at least $15/hr for child care; in fact, you can't even speculatively take a lower-paying job for career advancement if that job doesn't pay enough to offset child care.
Sure, if we want to subsidize stay at home parents for young children that's a reasonable choice. But, pushing daycare out of reach of most low income family's pushes people into poverty which also has significant long term negative impacts on those same children.
Results are best when there is a single primary care-giver and a small number of other regular care-givers, and day-care tends to not allow for this.
Things get much worse, however, when the staff of the day-care is either too few or too neglectful, as human contact and interaction is so important for early development.
It is possible that we could create a sufficient facsimile of a human to allow for automated care of a child that would fulfill these needs, but there is a creep factor in that, and there remains the question of how one would ethically test the efficacy of such a system.
Here in Australia child care is also very expensive, and I agree this is a problem for disadvantaged families (there is some means-tested subsidisation but it only helps to a degree). However: the government has recently lowered the required educator:child ratios, despite the extra cost, because research shows it is important for education (and health, not just when young but into later life). More than this: the research suggests these improvements are most significant for disadvantaged kids.
For more info start here: http://archive.acecqa.gov.au/research-and-publications/.
By law (usually state), the ratio of caregiver to child is much lower when the child is under 5. There's documented rationale for it too. Also, there is a ton of research now suggesting the importance of education at that age as a function of interaction of words with parents. So to suggest that "small children are better off exploring the world on their own, and making social connections on their own" is IMHO a very dangerous conclusion without understanding all of the implications associated with it.
 - http://cfoc.nrckids.org/StandardView/1.1.1
 - https://www.versame.com/research/