Page 20 ("ご飯にする" / "何が食べたい")
So I think just using the にする would usually mean “to have decided on”, i.e. here “since I/we have decided on (having) dinner shortly”. So I think it wouldn’t be a “have to decide on what to eat for dinner”, it would be a “decide on having dinner itself”.
But more specifically though, ご飯にする seems to be phrase essentially meaning ご飯を食べる - “to eat/have a meal”. That’s the answer given here, and it seems to be something someone’s mother said to them when they were young in a situation that seems to be similar to asking “Should we have a meal?”.
Also, here’s an example sentence from Howl’s Moving Castle with a translation (found here):
みんな！ ごはんにするよ！
It’s time to eat!
Damn, that phrase is hard to google thanks to a certain stock phrase…
As a side note - I once asked a friend about ーたい, and here’s one of the things I got out of that:
So it’s possible this whole thing has a kind of motherly/wife nuance.