90 days visa and 60 days extention is the max without marriage or court approval. You are correct that you used to be able to get a multi-entry Non Imm O but that hasn't been issued for a half Thai child for quite a few years now.
You can get a single entry Non imm O visa based on having a Thai child with just your name on the childs birth certificate, no need to be married or have gone through the courts.