In my opinion you've made entirely the right decision to re-home, given the age of your child, and the availability of suitable rehoming. Sorry this has happened though - must be very hard, and you have my absolute sympathy!
Re your walker, I do think you are being unfair in blaming her for not talking to you about the behaviour. (And I sincerely don't mean to be harsh, I totally understand you must be very upset at the moment, and looking for somebody to blame - that's only natural).
But in reality, hindsight is a wonderful thing. I presume she noticed certain behaviours in your dog around the container, but did not see them as especially serious given she was not bitten before, and it is only now that she is linking things up in her mind and realising that those behaviours were a precursor to the bite.
Dogs have very different minds to ours, and thus are even more unpredictable than humans. Cockers especially (I have owned and worked with a few) are more unpredictable than most. A cocker who was my beloved family pet, and under my direct supervision at the time, once bit a child also under my direct supervision - I take full responsibility and obviously, combing back through previous events, now feel I should have predicted it - but the reality is I DIDN'T (and I am experienced with dogs, reasonably risk averse, and not stupid). I am not saying I wasn't to blame - I was - and in the aftermath I spent hours combing over all the clues there were that the situation wasn't safe - but I also know with my rational mind that the dog biting was the last thing I expected. (The child was fine, luckily the dog's teeth barely connected and only grazed. Nontheless, quite rightly the event has stuck with me to this day, and is never far from my thoughts when any issue around dog safety is under discussion).
Given you have insurance, I would stop worrying about the legal aspect. Even the best dogs are unpredictable and can cause injuries in certain sets of circumstances - we know that when going in to ownership and make the calculation that the benefits outweigh the risks. That's exactly why we have insurance!
Nothing you have said makes me think your walker was especially negligent - it was just one of those things. Unfortunate, and especially sad for you to lose your beloved pet, but when you think about it the outcome could have been SO much worse.
I would strongly advise you to wait to get another dog till all the DC you plan to have are upper primary age. That's not a personal criticism at all - I just always think it's safer all round. And especially if cockers are your preferred breed, as in my opinion they are harder to read than some other breeds. Though let's be honest, no breed is 100% child-safe. And I proper love dogs!