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.