I think they started with a 50/50 scedule for loloking after the kid.
But after a while Tom started to remember the lyrics from an old Rolling Stones song, "Sittin' On A Fence":
Maybe the choice you did wasn't really right, then you go out and you won't come back at night
So B'elanna hade to take over the duties. But she was too fond over her job to quit Starfleet so in the end B'Elanna's mother Prabsa or Miral or whatever her name is got the responibility to take care of the kid.