You'll be in no mans land. If you get refused entry then Thai Immigration may impound your bike or send you back to Cambodia with your bike. The bike won't be a reason for Immigration to let you in the Country.
You need to go through the Courts to obtain sole custody of your half Thai child in order to obtain a 1 year extention of stay/visa based on being the childs guardian
You wont get the 1 year extention of stay or the multi-entry Non imm O visa based on having a Thai child unless you've been through the court process to be a legal guardian of the child. You should be able to get the 90 days Non Imm O in Savannakhet or Vientiane with just your name on the childs birth certificate. You can also get a 60 days extention of stay if your name is on your child's birth certificate on any entry into the Country.