If you mean me (your posts are very confusing as you don't refer to anyone when posting), I don't think I am missing the point.
Yes the headteacher has admitted they forgot your daughter was coeliac. But the school thinks it's no big deal, which is suggestive of them not realising/understanding the severity of coeliac disease. If this had been nuts, I think you would have a very different response.
The school clearly don't have the knowledge of coeliac disease to be able to risk assess properly for activities involving your daughter. This is a safeguarding risk and they need training. You are not wrong to complain, and to be honest I am very surprised at the dismissiveness of the response - the Headteacher is not coming across well here at all - but in order to move forward, you need to ensure they are informed while also complaining, otherwise they may make all the right noises about having updated risk assessments and so on, but nothing will have actually changed in terms of their ability to keep your child safe if they still have no idea what coeliac disease is.
I would write to the Chair of Governors, with something along the lines of the following:
'I am writing with reference to the Headteacher's decision to use a biscuit in a 1-1 lesson with my daughter last week. My daughter has coeliac disease and needs a strict gluten free diet, of which the school are perfectly aware. The Headteacher has admitted she had forgotten my daughter has coeliac disease, but I have had no apology or recognition of the seriousness of this incident. As such, I am escalating my complaint to the governing body; the lack of risk assessments and control measures around allergies is a serious safeguarding issue, leaving children at X school at risk. This cannot continue.
I appreciate that the lesson involving the biscuit was meant in good faith as an engaging activity, and I also appreciate that the Headteacher may not be aware of how serious coeliac disease is. My daughter's condition means that ingesting any form of wheat - even in the tiniest amount, such as a crumb - causes agonising pain and other distressing physical symptoms that could keep her off school for a week while she recovers. As such, her food intake needs to be very carefully controlled to ensure she stays healthy and pain-free. I therefore need urgent reassurance that an incident like this involving my daughter being offered and exposed to non gluten free food will never happen again at school - not just for the safety of my child, but for the safety of all other children at the school who may also have allergies. In a different scenario, a mistake like the Head's could have been fatal.
I rely on the school to keep my daughter safe while in their care and at the moment I cannot trust the school to do this. I enclose information about coeliac disease. I would like all staff at the school to have training so that they understand my daughter's needs and can ensure all activities are properly risk assessed in the light of these. I am happy to come into school to deliver training to staff on coeliac disease if required. I would also like to see a copy of your risk assessments involving use of food at school to ensure that you have adequately factored in my daughter's needs.
I look forward to your response. If I do not receive a satisfactory response within 7 days I will refer this safeguarding incident to the LADO myself.'
Good luck. Don't let them close ranks - if you don't get anywhere after the offers of training, etc. contact your local authority safeguarding team. That will get things moving.
Out of interest though, I would appreciate a response to my previous question - what do they do to keep her safe at lunchtime and break time? Many schools are now nut free, but I've never heard of a gluten free school - so how do they avoid cross contamination if everyone sitting near your daughter is eating sandwiches?