Have you booked which hotel did you go for? All three theme park hotels have one large buffet restaurant for breakfast and evening meals & you use your own one. You are given a recommended time to go, so that they don't get too busy with everyone one turning up at once, however they didn't seem to be too strict on times.
You are a bit off peak so should be fine. I was there over Aug bank holiday. The food is good for 4* buffet fayre.
If you want to use one of the other hotels buffets, you go to your own hotel reception prior to 6pm on the day you want to change, they give you a card to take to the other hotels buffet when yoou want to eat. (this stops people having three dinners
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one at each hotel). Each hotel has at least one a la carte restaurant with waiter service should you want to treat yourself, you get a monetary credit towards this & pay extra on top. We were happy with the buffets as we were 4 adults & 4 teens so had two tables for 4 each night.
TBH we didnt have much lunch as it was very hot. The breakfasts were good, we used to fill up then & just have crisps, ice cream, soft drinks in the middle of the day.
We did use to eat at 7.30 when the buffet opens, The food seemed to look nicer when lots of folk hadn't tucked in.
If you wish to buy lots of crisps, snacks, water etc you might be better off going in to Salou at the begining of you holiday and stocking up. Always supposing you can find somewhere to store it in the hotel room. (dont remember a fridge, perhaps you hire them?).
We managed with bits and bobs from the hotels pool bar (all three hotels have one) and gift shop, plus snacks from the theme park if we were in there at lunch. That was for 7 days not 11 and we didn't eat loads. If you are big eaters or have little ones who drink lots of pop this might cost a fair bit.
HTH's Doe