Here is a website that gives some of the con's of an invisible fence.
www.dogpatch.org/doginfo/invfence.html
I know my sister did it on her ranch in Wyoming for her 2 Pyrenees and she did extensive training with the dogs. At the end of the day she spent a great deal of $$$ and it doesn't stop her large dogs.
I would also be worried about how much shock these little guys could take. It probably would cost you a whole lot less to have a run built for them if you are not outside with them but I am not sure of your situation.