No you don't have a way to legally end the lease. If you want to move out you will have to break it. Its not the landlords fault that your family increased, and while its understandable that you guys need to leave, the landlord can't be held responsible for you having more kids.
Your best bet is to sit down with the landlord and explain why you need to leave. There will however be consequences for breaking your lease. If you have an early release clause then refer to that. If not then you could end up liable for rent for the rest of your lease. However if you talk to your landlord you should be able to work something out. Expect to at least lose your deposit.
This site has more information http://www.tenantsinfo.com/breaking.html

