[shaftway]

I separated the cost issue so it wasn't lost in the other wall of text.

I use a DCFSA (https://www.fsafeds.com/explore/dcfsa) to pay for it. Au pair expenses are a valid expense for this. Our marginal income tax rate is around 35%, so this helps a lot.

  Without DCFSA:
    Pre-tax Cost:  ~$32,300
    Post-tax Cost:  $21,000

  With DCFSA:
    Pre-tax Cost:   $21,000
    Post-tax Cost: ~$13,650

This is all an estimate of course (like increased insurance or rent on a larger place can't be paid for using a DCFSA), but it makes the point. So I think of the cost as around $13,650 for 45 hours / week, 50 weeks / year, 2250 hours in total. No matter how many kids you have.

If that big chunk is hard to swallow, the agency fees are ~$8000, and most agencies will let you break that up into a payment plan. The au pair is paid $200 each week, so that ~$10,000 is naturally spread out. The remainder is what we estimated our additional costs to be, like food, insurance, etc. and that gets spread out as well.