It is amazing how little we know on some areas and yet we use strong words such as illegal aliens when we refer to our clients, but that's another subject. An ITIN is for a person who crossed the border, got a towel, got himself dried and started working without the proper documentation and wants to file his/her taxes.
Going back to the question at hand, this is what you need to do. First, limit yourself to prepare their taxes geting them the biggest refund legally possible or reducing their tax liablity as much as it is legally possible. If they want immigration advice, they will consult their immingration attorney.
We know that not paying child support will not prevent you from claiming the child's exemption in 2013 as long as the child lived with him in the same household more than 183 nights. So you need to file as single for 2013 and if he qualifies, he can claim the child and if he further qualifies he can file as HoH.
In 2014, they are both your clients and you can talk to them about the fact that if they are getting a refund, it will be redirected to cover his child support if he is not current. The wife and him have only two choices to file: MFS or MFJ. You will file 1040 all along.
If you they decide to file jointly, you list her first on the return and list the husband second with his ITIN. Since he has been here in the US, you do't have to do anything special. 99% of these people file jointly because that helps to prove that their marriage is a bone fide marriage but that something that is beyond our scope.
If they decide to file MFS, then you will prepare 2 1040 and that's it.