What documentation do I need for my spouse and baby to apply for a Non-O visa in Thailand under my Work visa?

Elani -- just for little clarification. The B Visa is for Business (not titled 'Work'), however it is intertwined with a Work Permit. In most cases the Non Immigrant B Visa is issued by a Thailand Embassy/Consulate. The Thai Labor Ministry issues the Work Permit. In the B Visa application process a person must have at least proof of a successful Work Permit Application - usually a type of receipt called a WP.1 or WP.3 ... This WP.1 or .3 Application / Receipt will generate a Labor Ministry Official Letter. These Labor Ministry documents must be attached to your B Visa Application at the Thai Embassy / Consulate outside of Thailand where you apply. There are other documents that must be provided too but mostly are related to your employer's company. You will supply copies of all pages of your passport in the B Visa Application with is for a Non Immigrant B Visa 90 days and proof of your University Education. After award of the B Visa you can later get an Extension of Stay based on the B Visa or you can get a Multi-Entry B Visa for one year which requires a Border Bounce (Border Run for an out and in stamp every 90 days. These last two options vary depending on how your employer handles the situation.

There are some variations on this entire B Visa / Work Permit process - but the process I just described is a very common method and sequence used.

Also - if others have corrections to my comments - then I will be pleased to stand corrected.

honestly if the baby is super small I wouldn't even fool with getting them a visa. Just let them overstay :O I know that sounds bad but children under 15 can't be charged for overstay, and children under 18 can't be banned for overstay either..

Are you going to apply for a Non Immigrant B visa at a Consulate or Embassy outside Thailand or are you going to apply for an Extension of Stay based on working at you local Immigration Office. There is no work visa.