I think any of the hospitals that have dentists would have them. (???) Try Bangkok Hospitun, Lad Prao Hospitun, Praram 9 Hosp. There's also Bangkok International Dental Clinic BIDC Thailand Dentists - Bangkok Dental Clinics and Bangkok dentist
Call them up to see if they treat kids. I've used the dentists at Lad Prao, Praram 9 and at BIDC. I don;t have anything wrong, I just go for my cleanings every 6 months. All were very good and the cleanings were between 800 and 900 baht.