With not too much searching we found a building housing a company named "Music Skills" and "Artino School of Creative Arts".
The music school offers classes after normal school hours and on weekends. All normal instruments are on offer. My daughter is taking piano at 5,000 Baht for 12 hours. She goes 1 hour a week on Sundays. They have recitals and concerts for parents each term. The biggest plus so far is that she's learning how to read a sheet of music and apply it to an instrument. That can't help but be beneficial to her later in life.
The art school offers everything up to portrait painting and western art appreciation, along with other types of art training and practice for the younger learner. 2,800 Baht for 8 sessions of 2 hours each.
The music and art lessons are both in Thai though so the child will have to have at least basic Thai communication skills. In turn, a Thai child needs some English as much of the art appreciation material is in English.
My point in posting this is that if there are worries that your child here is not getting the degree of western cultural training that you would like, places like this are here. You just have to search them out. Pete
Pete