Shiney, just as you realise that your puppy 'nipping' was just puppy mouthing and completely normal for a puppy until they are trained otherwise, so are some people recognising that nipping can be perfectly normal behaviour for some collies until trained otherwise. No difference IMO, a puppy is perfectly able to do damage by mouthing, just as an adult collie can.
One thing that some people often fail to understand is that being aware of dog behaviour, having an idea of how to deal with certain behaviour and being willing to do so does not mean people are putting dogs before children. What it means is they believe there is a way of solving the problem whilst keeping both the DC and the dog safe. I have said before and I'll say it again, I have a dog here who previously was at risk of biting someone. I also have DC. However, my DC, nor anyone else were at risk of being bitten, because of how the dog was/is managed.
I think the OP is right to seek professional help, good help. It seems they have had the best intentions and done that immediately, but sadly the trainer is lacking and that probably hasn't helped.
I do think that maybe the breeder here has been given a hard time too. I personally (as a breeder) wouldn't agree to a months trial of a dog, although I would make it clear that I would take the dog back at any time in the future and in fact, expect that to happen if the new owner cannot keep the dog, but I don't get the impression that the months trial means the breeder won't be interested after that. The OP herself has said she hasn't contacted the breeder as she is worried they may take it back, not because they don't feel the breeder would help. There are also valid reasons as to why a breeder may have to rehome a 12 month old dog. It may have been returned to them and they are not in a position to keep it, or they may have grown it on for the ring and it has turned out not to be good enough to show or breed from. Not something I would do myself, but I do understand why some breeders so this and provided the new home is a good one and the breeder offers lifelong support, I don't see the problem with it. Of course, the dog's behaviour and the fact it has been rehomed to a family with a young child does not seem ideal. I am not a collie expert (although I am not sure fi some people have missed the fact this is a rough collie, likely show bred and now a border or working collie) but I do know myself that it is possible for dogs to exhibit certain behaviours in an inexperienced household they simply don't when in a more experienced one, maybe for as simple a reason as the atmoshphere. I knwo my own dogs are super obedient, but they do know when people are a 'pushover'.
I think waiting to speak to Wiccaweys is good advice, but I would also urge you phone the breeder too and discuss the behaviour with them. They should be experts in the breed and may well be able to help you in the meantime. I wouldn't be too quick to write them off and they should be glad you have contacted them and commited to finding a solution to the problem, whatever that may be.