My wife''s ex-husband is required by divorce decree and AID to pay 50% of College tuition on his 2 kids. He has done this on first child for last 2 yrs

Please talk to a family lawyer in your area and bring your Decree and AID with you for review by the lawyer.  There is the option of trying to resolve this case collaboratively instead of through litigation which is a wonderful way of resolving disputes. Talk to the family lawyer about using collaborative law as a means of settling the issues. Most likely, if you do not want to file suit or there is not an agreement between the parties to try the collaborative approach, you could have your lawyer write a letter to the ex husband demanding he comply with the terms of the prior Order and agreement.  There might also be a mediation clause in the documents thereby compelling mediation first before filing.