I am just telling you what immigration told me. I’m happy for you to discuss it with them. I don’t make the rules. I travel to Thailand 12 times a year for a week at a time. They told me 90 days is a maximum without a Visa. They told me to apply for a type O non-immigrant Visa.
60 on arrival, 30 day extension and 7 day more to leave the country. You will get another 60 days on arrival only if you can prove you are a  genuine tourist. I just hard this conversation with the immigration officer when leaving the country on Saturday.
You may need a letter from their parents and a copy of their ID giving permission for them to travel with you. Better to have the correct documents and not need them..