The suspensions are not really a surprise. The first Labour King's/Queens speech in 14 years, the expectation is that nobody votes against them. Abstain, sure. But to vote against your own governing party gets the whip withdrawn.
Politically, I think this issue is more nuanced. There are a lot of people, including a lot of left wingers, who agree with the cap. They go along with the notion that if you are living on benefits, the state should not support you having loads of kids when working people limit their families according to their budgets. But as @aDONisSheep points out, that isn't always the case. Not everyone starts off in that place.
That said, I saw on the BBC news a couple with 5 kids who have never worked, complaining about it. The reason they can't work, they say, is childcare.
The cap will be withdrawn this parliament, I'm sure of that. At some point it'll be done as a big announcement. But the SNP have duped these MP's into changing the story of the King's Speech into this. You have to say well played on that score.