My 3 and a half year old has never slept through. She wakes between 12-1.30 am then generally an hour later, then wakes at 5am and there's no chance she will go back to sleep.
We follow a good night routine, have a dim nightlight, warm milk, book, bed at 7pm.
When she wakes we don't bring her into bed with us or anything. She doesn't just stir and cry a bit so we can ignore and see if she goes back to sleep- she begins by screaming really loud and repeating 'I want mummy' over and over. Dp always has to go because if I go she'll never go back to sleep. Plus at the minute if I get up through night my morning sickness and dizziness kicks in ( I am 7 weeks pregnant). We also have a 5 year old who also wakes generally once through night. We've tried putting them in room together for comfort but that didn't work either. We've practically given up on sleep.