If the children are of different genders and one is adolescent they would normally require separate bedrooms, so a three-bedroom house would be needed.
When I was very young my Mum and dad bought their first home and it had three bedrooms. Lucky me got one to myself because the three brothers had to share. When we moved here it was considered the height of luxury to build a four bedroom house with two toilets no less. Two brothers still shared.
Many - if not most - of the families discussed here are sole parent. If I was the sole parent with a low income I'd been opting for the smallest home in the best neighbourhood - maybe even one bedroom if the children were very young. The idea that a single parent with one boy and one girl automatically requires a three bedroom home is silly. But it does help the case of those who want to push up costs as high as they can.
I am not denying that low income families struggle. But the inevitable conclusion that government has to direct more money into these homes is not a given. You can argue the moral case, the economic case, the political case.... But perhaps the worst aspect of this type of thinking and analysis is it takes away from the poor the expectation that they can be smart, adaptive and innovative; that they can work out for themselves how to make their lives better tomorrow by making some sacrifices today. It's thoroughly patronising and paternalistic and will not deliver some sort of socialist nirvana. It will result in a helpless, dependent population who one day wake up and realise that the all-giving state is now their worst enemy.
Strike that. It's already happened.