As you never gave the age of your children, it's difficult to answer correctly, but I am sure the Tai Pan hotel on Sukhumvit soi 23, will accommodate your booking.
Having lived in Sri Lanka for nearly 20 years ,l find Thailand Cheaper for most things,a long resident working visa is extremely difficult to get and now very expensive in Sri Lanka.
He does not need an Agent,as long a Thai Bank account with TB 800,000.00 in it or his pension details that equal the financial requirements,then he can go to immigration and apply for the O visa himself, instead of getting ripped off by an agency firm.
I obtained my O Visa within 28 days,so 30 days is enough time.