Can I change my marriage visa extension to a Thai child-based extension without leaving Thailand?

There shouldn't be an issue changing your reason for your extension from one based on marriage to a thai to one based on raising thai child.

Go talk to your immigration office well before your current extension expires. SOME offices won't let you change if you're still married (but will let you change if you're getting divorced)

You have to check with the office you use. Also keep in mind while technically there is no seasoning required for the raising thai child extension SOME offices do require the same seasoning as a marriage extension (400K baht in a thai bank account for 2 months before you apply)

That's why I said go talk to your office well before your next extension. ..