We did this and came out just fine. 8 Years ago, we went from earning 80K (me) and 22K (DH), to 24K (just DH) after our first kid. Kept that up for a little over a year. We basically lived on his income when we were both earning, so it was 'easy.' I was very, very proud of that year. After a year, I went back to work, earning 80K, and he became the SAHP. We had another kid and moved into a bigger place. (necessary, IMO, we were previously a family of 3 in a 399 sqft apt., 4 would have been way too tight.) Our expenses crept up, yes, but not unreasonably so. These days, he is back to work part time, and we try to save about 1/2 of our income. I haven't calculated actual rates in a long time, and we spend some of what we save in the previous year on fun stuff, so that's a really rough estimate.
So maybe you can't keep up the savings, but just maintain a frugal lifestyle, and it will all be good. Our net worth was $190K when we had our first kid. Today, our kids are 8 and 5 and our net worth is $733K. Not too shabby. Also, my lost year of 401K contributions when I wasn't working was a bad year to miss - great "sales" on stock in 2009...