I think it's a reach to assume that SS should have passed threshold to remove. There's a process you can't just whip a child out, you need grounds to do it. In this case I'm assuming people mean her failure to meet his medical needs, well you'd need the hospital and community nurse to concur with that in the first place to make these referrals.
There's also the likelihood that they may already have a Social Worker via Children with Disabilities which may explain CC being involved with O having a Community Nurse and the boys remaining at nursery, she may have had funding for that as a support to her.
There's plenty of other professionals involved, if they had concerns about the children they'd be quick to refer in, you have daily contact with Sienna's school, the boys' nursery, HV, paediatrics, community nurse. The health side and school are well versed in dealing with safeguarding concerns and if there were genuine concerns I would be very surprised if these bodies hadn't referred in.