Starting "big school" is a HUGE milestone for both parent and child, but as a parent one aspect you may (or may not) have noticed yet is how tiring a school day really is for your little one, and what this does to their appetite. When children come back from a full day of school, their tummies are usually starting to grumble because if you think about it, they havent eaten since lunchtime, which is probably close to 3 hours ago by the time you pick them up and get home!
One of the best things I implemented when my kids started school was snacks and early dinners, especially during their younger years when bedtime was between 7 and 8pm! I started this becasue I found that the hungrier they got, the more they craved high-sugar/high-energy snacks and the harder it was to 'negotiate' with them to have a healthier option instead.
So to tackle this I would always have a light, healthy snack prepared for them when they got in the door after school pick-up, whether they asked for it or not, like a fruit salad or a variety of vegetable sticks, sometimes with a dip (if I had time to make one, or found one with a clean label). This would ward off any strong hunger pains before they could kick in, but still have them hungry enough to look forward to what was on their plates at dinner time, which was at roughly 5:30/6:00pm, without them reaching for sugary snacks in between.
This way they filled up on nutrition instead of sugar, and it meant that they were no longer starving and over-tired which used to make afternoons and dinner time so hard to get through, especially if there was something "new" on the table! I also found that they used to sleep better because they weren't having a huge meal just before bedtime because they were starving.
I highly recommend implementing somthing similar with your "big kids", it really does help make your afternoons and dinner times less stressfull and it creates healthy eating habits that will carry with them as they grow.