If you have registered marriage with your husband (current), then I think there would be no problem to get dependent visa for your son from earlier realtionship. Better to contact Thai Embassy at your home country for further details.
Need to go back to home country for Indian passport. There you get sticker visa for max 3 months, for which you will be given 60 days entry in kingdom. Near by any country you go and come back, you will get 30 days entry. No option for 6-12 months tourist visa in your case.
90 days report means, you complete 90 days continually in Thailand without leaving the period of 90 days. When you leave before completing it, no need to do anything. When you come back it starts again from the first day to 90 days circle.
Just go to airport...while immigration counter, officer will ask you to pay fine 500 per day. Pay your fine, get a receipt and depart to your country..but beware to be caught before.