Congrats on new baby!
I'm a working Mom of 2 (2 yrs old and almost 1 yr old). I love what a previous poster (Theresa) said about future earnings. I think I am a better Mom because I get that little break. I am not trying to advance my career, just trying to maintain for a few more years until I retire. Plus, I have heard from other friends & this forum that kids need you MORE when they older for guidance & activities, so I want to just get this time of working over with.
It's nuts with both my husband and I working with 2 small kids, a house, and a dog. Every day that we work, I say a little silent prayer that we made it another day, adding more to our net worth and everyone is alive and doing well.
If you are concerned about expenses rising, I would seriously look at sheltering as much of your income as possible (traditional IRA, 401k, health savings account, etc). It sounds like you guys are getting by on his income, so this would help you with future earnings, as well as getting more set aside for retirement & such. RootofGood has some good articles on how they make $100k a year and virtually pay no taxes because of all the sheltering they do.
Also, check if you can do a dependent care Flexible spending account to further reduce your AGI to help with tax and the various subsidies you get.