This kind of thing is terrifying to many parents and really, really doesn't help childcare professionals gain a rappor. It's also a often seen online trope which can be terrifying for anxious parents or those who worry about heavy handed HV:
I think that refusing a simple developmental check might be the fastest way to have the health visitor knocking at your door. Sometimes refusing contact or doing something out of the ordinary like refusing a standard check attracts them like flies to a dung heap. If they have had "concerns" then it's no as simple to say you don't legally have to see a health visitor as they can just refer you to social services.
In my area there are loads of kids who don't do their checks - variety of reasons including mummy and daddy don't do NHS dahling or they just don't engage with the HV in that way.
The child is at nursery therefore concerns would be linked to anything seen at nursery. The HV would not contact SS if the nursery said "oh smallchild is perfectly normal/clean/cared for, they're on holiday/can't make it..." they would have a telephone conversation with the parent.
This reminds me very much of the terrifying 'what happens if you go to A&E' shit which I found when DD was 8 months old. Of course being aware of my own anxieties I did still take her, then cleaned the house for 2 days solid fearing an inspection and declaration of unsafeness. SS did not come round. The HV did not come and inspect my house with a fine tooth comb. The HV popped a note through the door and followed up with a phone call - I surmise that as what I said tallied with what the medics said (child climbed onto sofa when cruising, child fell off onto face, no injuries, PFB) and I explained I had purchased a play pen, they were fine with it.
HV-snitching-terror is probably more dangerous than anything as someone really scared might disengage more to keep off the radar (e.g. avoid A&E if actually needed, avoid free nursery places because of stuff like this...).