@Sophie ******
, You could probably apply for a Non-O single entry visa from the thai consulate in the UK using the eVisa online system based on you holding a Non-B visa/extension and a work permit and you being the mother of the child
AND
then when you get here they'd get stamped in for 90 days and you'd go to the immigraiton office when you had 30 days or less left on that stamp and the child would get a matching extension of your expiration of your extension that you have based on employment