What I've been reading: Sarah-Eve Farquhar
I'm not saying that this author has all the answers, nor that I agree with everything that is said in the full article. But I think it is worth reproducing her overview which you will find in that link (with my emphases) here:
Early childhood education/childcare policy in
The best evidence points to parents/family having a far greater impact than the childcare/ECE experience on children’s developmental outcomes. This suggests that effort put towards supporting families to provide great home learning environments for children and allowing parents to make child-rearing arrangements that are best for their child and for family well-being would pay off. Such efforts would more likely make a greater difference to children’s immediate and long-term developmental outcomes than simply encouraging parents to make more use of non-parental childcare as present ECE policy does.
Children attending full-time ECE/childcare as compared to part-time (around 12.5 hours per week or 2.5 hour sessions) do not have significantly better developmental outcomes. In other words there is no advantage to be gained for children attending non-parental ECE for longer hours than parents require for childcare. This suggests that the way ECE policy financially incentivises early childhood services to increase the length of sessions and to promote 20 hours or 6 hour days as an optimal minimum, is unnecessary in terms of any added benefits for children. (It may also be at the cost of children not accessing formal ECE still not being able to access it).
The evidence also points to ECE/childcare having both developmental risks and benefits. While there may be cognitive gains (at least in the short-term) there can also be negative outcomes for children’s health, mothers’ sensitivity in interaction with their children, problem behaviours and aggression in children. Public policy that emphasises telling parents about the benefits of formal ECE and not about risks and other childcare options may increase the risks for children further. Enabling parents to be informed and to make informed choices would put parents in the position of being able to better manage risks and understand the size of potential benefits of ECE in interaction with home factors and later schooling choices and experiences.