It is absolutely the nursery’s responsibility to look after and keep safe all the children in their care. That is what parents pay for. They don’t pay other toddlers to regulate their own behaviour.
Nurseries can only take-on the number of children they can manage to do that for successfully. They need to factor into their calculations that at age 2 or whatever, some kids will need greater supervision than the standard ratios and then to provide it as required. It is not good enough to say that the official ratios were met, if they did not keep children safe.
Nurseries with toddlers will always have some biters, some hitters and always some unpredictable behaviour. Their job is to know this and to provide adequate supervision. The reason nursery is so expensive is because of the high levels of supervision needed. If a nursery cannot make a profit and meet the necessary levels for the legal requirements AND the individual children they have in their setting, they simply cannot operate. It isn’t acceptable to know you have a child who has injured another child and to not then adjust the ratios and staffing and supervision to significantly minimise the risks in future.
So, nurseries don’t always know the risks of individual children until they emerge. That’s why there are minimum ratios and best practices. You don’t leave children in a room alone, so even if they are very well behaved and mature, on the occasion when something WILL happen, and there’s no doubt it will, you are there to act speedily.
And when a specific risk appears - and again it will, in terms of a biter or a hitter, or whatever, you then have to have the flexibility and staffing to UP supervision in those areas where needed and reduce the risk.
These 2 things - the proper measures in place all the time, plus extra measures when needed are what parents need to know exist, in order to be able to leave their kids at nursery. The nursery can’t guarantee there will never be a biting incident. However, appropriate supervision should mean anything is quickly spotted and stopped. And then extra measures put in place to reduce the risk in future. That’s what parents can expect.
If a biting incident and reporting happens as described by Op, there wasn’t adequate general supervision to start with, plus even more worryingly, no immediate response to adjust supervision to minimise the risk if a repeat. This is unacceptable on both counts.