Wow so many messages and nothing yet answering the OP’s actual question 😂
OP, assuming you are in England:
Your child reaches ‘compulsory school age’ (CSA) on the first day of the term after she turns 5. So, if she is summer born she is not CSA until the first day of year 1.
When a child misses school, the school is legally required to put an absence code for her on the register. Before she is CSA, they use a code that means ‘attendance not required because child is not CSA’. That code can be used an infinite number of times, she could even miss all of reception year and there would be no legal consequence for you or the school. We chose to do half weeks until my child was CSA. (Why? Because I felt like it, and because she learned more at home than at school. We only sent her to school for the social side.) Your child is not yet CSA so you can ignore this letter. As she is below CSA school shouldn’t really have sent you the standard ‘absence letter’ at all, but often school staff don’t understand the rukes.
After your child reaches CSA, things change a great deal. She must be either home educated, or registered at a school. If she is on the register at a school, then you are legally required to send her to the school site every day (and it is a criminal offence not to) unless the school authorise an absence. They have a list of specific situations where they can authorise absence. The main situation is illness but they can also eg authorise children to be absent because on a school trip.
So, after she is CSA, when you tell school that she is ill, you are requesting that they authorise an absence. Usually they do this but if the child is ill a lot they may stop believing you and refuse to authorise, in which case you get fined (£60 per parent per half day session).
In general just be charming and friendly with school even when they’re being dicks, it is so much more helpful to you in future. If I was in your situation I would write back saying something like thanks for your letter, yes we are all very sad that DD has been so ill this term and we hope things improve, we don’t need any support as we are in touch with our GP about DD, we’ll let you know if that changes, in the meantime please do ensure that her absences are recorded on the register with the correct code for a child who is below compulsory school age and thus not legally required to attend all sessions, many thanks.
Hope that helps.