When you get a mortgage, you have an obligation to pay it. An increase in family size does not justify walking away. It seems like everyone is looking for an easy way out. Everyone else is going through foreclosure, so it's OK if I do too, right? Wrong! Foreclosure should be avoided if at all possible.
You didn't say how many kids you have, but you have a 3-bedroom house. Guess what? You have one bedroom for and your spouse/partner, one for the girls, and one for the boys. Sounds great to me. My husband and I currently have two bedrooms and three kids, so I know what it's like to live in a house that's too small. We're looking for a bigger place, but for now we're making it work.
To answer your original question, you can expect your credit score to drop by about 200 points immediately if you go through foreclosure. You will probably not be able to get a loan to buy a house again for at least two years. Your ability to qualify for other loans and credit cards will be affected as well.
Another thing you should think about is the message you send to your kids if you walk away from this house. By doing that, you are teaching them that your word means nothing, and that it's not important to stick to the agreements that you've made. A lot of people are having a hard time financially right now. That doesn't mean they shouldn't at least try to do the right thing.