Anything at all IMO. DD2 had severe tonsillitis 3 years ago & didn't eat or drink for a week. Custard, icy poles, yoghurt.

More important than food though, is hydration. Doesn't matter what they drink, although water is best, as long as they are drinking. It's so painful to swallow that you might have a better chance by using something like juice, or cordial. Purely to get those fluids in. Ice, icy poles.

I stress a bit over tonsillitis, after our experience DD was in hospital for 5 days & I was bribing her with everything under the sun. Milkshakes, lemonade, coke. I tried everything & nothing worked. She also had glandular fever at the same time, so pretty full on.