please count from the latest (last) date you entered Thailand. From there if your continuous stay is 90 days or above then 90 days reporting is required. If your continuous stay is not 90 days and you plan to leave thailand then 90 days reporting is not required.
What is the status of the parents? Thai foreigner or foreigner - foreigner? But as Tod Daniel replied, you dont have to worryvmuchbif the kids are in tgosevage range
usually most of the Banks dont entertain if you are on tourist visa or visa exemption. They may ask for the letter from your embassy. I had shared information to your inbox. please read that. Hope that may help you