I've looked at this in detailed today as I was not sure myself what was the policy.
The project has what is called 'pertinent or primary' finding, which is a results that is 'relevant to the explanation, main diagnosis or treatment of the disease for which the patient was referred'. So in your daughter's case, anything directly related to her developmental delay. These results will be fed back to you.
Then it also has what is called a 'secondary finding' which is an 'additional, looked-for health related finding, that is not pertinent to the main condition.' These are findings that are 'known to cause serious conditions for which there is good evidence that knowing about them could influence healthcare'. Most of these are cancer genes. I looked at the list and the common genes for breast cancer are there. However, it also says that 'only the mutations in genes that are known to cause childhood-onset disease will be looked for in the case of minor participants". So in your daughter's case, if the breast cancer you mentioned runs in your husband's family (which I assume was developed late in life) has a genetic cause (which is very likely) this will not be looked at because it will not affect her now. But, I should also say, that if they find a 'faulty gene' that is known to cause childhood onset cancer, this will be fed back to you.
Hope this makes sense. You can see all the information here (page 42)
https://www.genomicsengland.co.uk/wp-content/uploads/2015/03/GenomicEnglandProtocol_030315_v8.pdf
The issue of how to deal with incidental findings is a very difficult one, and there is lot of debate about it, both in the public and the healthcare community. Some people argue that they would prefer not to know, yet if it is a life-threatening but treatable condition I would say it is better to know.
Let me know if there is anything you want clarified. You can PM if you prefer