How can I obtain a Non-O dependent visa for my child born in Thailand without leaving the country?

Depends on what your real intention of getting a visa for the child is.

As

@T**

already mentioned, a Thailand-born foreign child cannot get an actual visa or extension until getting an entry stamp after leaving Thailand once.

What difference between Thailand-born foreign children (with the correct ID/passport and Thailand birth certificate) and visiting foreign children is, that former can stay in Thailand legally indefinitely until leaving Thailand for the first time or hitting 15 y/o (or maybe 10 y/o, that is I don't know exactly) while latter will be overstaying once pass the date on entry/extension stamp (regardless of being fined or not).

You need to check the details about this with your local immigration office though.

What is the status of the parents? Thai foreigner or foreigner - foreigner? But as Tod Daniel replied, you dont have to worryvmuchbif the kids are in tgosevage range

Short answer is you can't. 😮

There's no way to get a visa issued inside thailand for a foreign born child until you exit/re-enter and get them on a free 60 day visa exempt entry stamp.

Once they have that, then it can be changed to a Non-O based on a parent working here legally,

but

'no-stamp' children (born here who haven't left) can't get visas issued.

There is no real rush to get them on a visa, because children under 15 y/o can't be fined for overstay and children under 18 can't be banned,

Having them stay here without an entry stamp or visa/extension isn't gonna be a problem for years (depending on how old they are now)