There are two parts to your question:
How does your child qualify?
As other posters have advised, in Enhland (if that is where you are, apologies I am.incorrect) currently all children in EYFS and KS1 (Reception, Year 1 and Year 2) automatically receive free school dinners. This is the UFSM (Universal Free School Meal) scheme. As it stands, when your child goes into KS2 (Year 3) you will need to pay for school meals unless,prior to that point, you meet the criteria and have applied via your local council for FSM (Free School Meals) which are means tested and aso have implications of other funding for your child while in full time education (eg free school milk, possible findi g for trips etc)
How do I make sure my child gets a school dinner on the day they start school?
Unil your child walks into school on their first day they aren't yet on the school roll, and while not on the school roll can't be set up on school systems such as meal.ordering sysems, payment sysyens, school.apps etc. You will usually get invites to set up all these within the first week or two.
You need to check if your child is having a dinner at school fir the first few days... some schools, if doing half days at first, dont include a school dinner to help children not be overwhelmed on day 1. If they are, it may be children are going to pick their own dinners in school if that is the usual.option (with help of staff) to gain a little independanxe.
If you have been receivi g communications from school have a look back through - emails, parent apps, letters etc whete there might be a quick sentance that covers it.
If office staff are term time contracts, tjey wont be in until school term starts (sometimes day 1 is an inset day so you might get further communications the day before they start).
Good luck to your dc!