They have a duty of care toward your child, not providing sustinance or being complicit in the withholding of food is a breach of that. Governance procedures should be in place to ensure this type of incident does not happen. They will do their best to wiggle out of it by saying he didn't follow the procedure but as a seven year old he cannot be held responsible for his actions. I would settle for nothing less than a groveling headmaster/mistress and a firm assurance that procedures had been improved 
I can draft a letter for you if you like 
(Can you tell I work in an organisation that is highly beaurocratic?
)
I'm sure a word in the correct ear would suffice. I doubt very much if they intentionally witheld the lad's dinner.
Doesn't matter, governance processes have to be followed where failures occur... especially in the situation where you have a duty of care of legally vulnerable persons
My wife, headteacher, would listen apologetically to the concerned father, but would use your letter as toilet paper.
rather jobsworth.
I use cheap printer paper so I expect that the knowledge of the pain that it would have caused would be fine. But a nice apology woud have sufficed 
Listen, Matt, when a parent brings a letter into school that uses phrases like 'Governance issues', do you really think that they attribute it to aforementioned parent? Or do you think that they put it down to a 'helpful' solicitors clerk, who thinks that it will get results?
The correct procedure is to talk to a member of the senior leadership team and, if it's not resolved to your satisfaction, to ask for a copy of the school's complaints procedure. Once the CP is mentioned, the situation is usually remedied pretty fast, as the procedure is very robust.