Yes get a will. Or else someone taking funds out would be illegal. The bank release the money if they are stated in the will (not sure of the process).
I was referring to an annual extension, but you might need to apply for a non imm visa from outside the country and then you can extend it for 13 months, as I outlined. You would be best to contact your immigration office so they will outline what they require.
If your named on your child's birth certificate, you might be able to apply for an extension to "support a Thai child". However, I'm not sure what extra documents you might need to produce, best to ask your immigration office.