How come you expect your daycare expenses to go down in 4 years? Is DW (or you) planning to quit working once the student loans are paid off?
I'm curious--if DW quit now, what would the impact be on your finances? You'd lose her income, sure, but between FICA (7.5%), income tax (looks like your marginal rate is 34%!), state income tax (wherever you are), and daycare expenses, not to mention expenses for work (car-related, clothing, etc), how much of her $55k/year do you actually net?
It's fantastic that you're able to save $5k/month! However, $6600/mo for a family of four, especially with the kids really young, is rather high. I don't consider myself particularly frugal, but our family of 8, including teens, lives on a similar amount, and we live in a state where we're paying $10k+/year in property taxes.