As stated, this will look different for every family, and even in the same family, for any given stage of life.
We have 5 kids, and when they were young, we didn’t really give them spending money directly. Instead, we would tell them something like, “You can pick out one souvenir for the trip” and “you can choose one special snack per day.”
So, then as the week progresses, we would take pictures of anything they might be interested in so that they can see it all at the end of the week and make a final purchase decision. If we had the money to, we might supplement their decision by buying something else for them we know they especially liked. But it just depends on how much money we had available ourselves!
When the kids were older (say, 8, 9, 10 or older), we started giving them instead a budget. And once they were responsible enough to carry the money themselves, we actually gave them the cash (or in one case, a Disney gift card) in the amount they could spend on BOTH souvenirs and special snacks. (In this way, if they wanted to forego the snack so that could buy a more expensive souvenir, that was their decision.)
Now, they were always welcome to use any additional money they had earned on their own to supplement the spending money we would give them.
The AMOUNT we give them varied widely from any given vacation because it was based on how much we actually saved. We usually save for about a year for a given trip. If the actual costs of the trip comes to $3000, and we managed to save $4000, that means that $1000 was available for that trip for souvenirs. Part of that we allocated to the kids and divided it up. Some trips, the amount “extra” would be like $200, so that means very little money was available for souvenirs unless they supplemented the money themselves.