you can get Single Entry Non Immigrant O visa based on having Thai Children at several Thai Consulates in the area or even at the London Embassy.
You can apply for the 1 year Multiple Entry Non Immigrant O visa based on having a Thai Child at the Thai Consulate in Penang, but one of the requirements is 400.000 Thb or equivalent currency in a bank account on your name
EDIT:
@Tim ******
please share this information with him.