Getting daughter back for you: very difficult
For them to get son from you: very difficult
The addl complication is you are in US and they in India but it may also be in your favour in terms of son custody.. DO NOT let your son to go to them if and when you come to india, I can't repeat it enough times.. India is not signatory to hague convention on child custody... any order passed by US court on child custody to you has no meaning in India.
I think I have replied on your case details in saveindian family yahoogroup already. You can read there too.
For child custody, you have to educate yourselves on law & judgments. It is NOT based on truth or guilt of the husband/wife parties. It is solely based on where the child is set and adjusted in terms of education, emotional health, family support etc.
Read my blog there are many posts on click child custody visitation link on right: