Under the Labour Protection Act, employing anyone under 18 in a place that serves alcohol is explicitly prohibited. Working in an “entertainment venue” (i.e., bar) under 18 also breaches the Entertainment Places Act.
Also, whilst we cannot say with absolute certainty that alcohol was consumed by the minor in question, given that it plays a primary role in how these establishments operate (selling "lady drinks"), there's also the legal age for purchasing alcohol to take into account, that being 20 years old.
In addition, although the age of sexual consent is 15 in Thailand, specific laws criminalise sex with minors 15–17 years old within a prostitution or entertainment setting. Under the Prevention and Suppression of Prostitution Act, engaging in sexual acts with someone aged 15–17 in a prostitution establishment (bars included) is punishable by up to 3 years imprisonment and/or a 20,000–60,000 THB fine.
Then, of course, there's trafficking - specifically the Anti‑Trafficking in Persons Act - which states doing so to a minor for sexual exploitation can result in 20 years or more in prison, as Bilton chose to find out the hard way.
Edit: For clarity, by Thai law, a minor is considered to be anyone under the age of 20, unless they are married (the legal age for marriage is 17).
2nd edit: Since they're not the ones purchasing the alcohol, my second point may not apply anyway, but it's a moot pint really as it is illegal for anyone to sell or give alcohol to anyone under 20 years of age under the Alcoholic Beverage Control Act; and also the Child Protection Act states that anyone under 18 is a "child", and it's explicitly forbidden for others to sell, exchange, or give alcohol to them (except for medical reasons). Violating this can result in up to 3 months imprisonment and/or a fine of up to 30,000 THB.