If there was real, credible evidence that children do better with a parent at home, then I would be the first to argue that the state should do more to encourage this. However, the evidence does not support it.
And again, if there was real, credible evidence that there was a net benefit to society from having more SAHMs, then I would be in favour of policies that enabled women to make the choice to SAH. However, there is no real evidence of any net benefit to society.
SAH is a lifestyle choice that primarily benefits the SAHP (if that's what they want to do) and their partner (if they want that division of labour). It is an entirely valid choice to make if the family can afford it, and there may well be benefits for that particular family - eg less stress for both parents etc. However, this is a choice that needs to be funded by the individual family and not by the state.
What the evidence does show is the importance of under 2s in particular being able to form secure attachments with a small number of consistent caregivers. However, that doesn't necessarily mean that we need more SAHPs. It might mean that we need better funding for maternity leave, now flexible working arrangements so that parents can share care between them, and more investment in high quality childcare etc.