You will get a wide variety of answers on this. It all depends on your personal spending habits. We give our children enough to buy 1 reasonably priced souvenir of the trip ($20-$30). They usually "window shop" for the first few days then we make sure to buy whatever it is later in the trip. We also budget enough to buy a few special "treats" for the kids (for instance, if we see a t-shirt we think they'll like, we might say "do you want this?" and buy it for them. Their "allowance" is for whatever *they* pick out. They really think about what they choose and, for the most part, they are happy with their choices when we get home.) I don't think we spend more than $50 per child on the whole trip for souvenirs though.
Other people give their kids $100+ per *day* for souvenirs and snacks.
$20 per park would buy them a small/medium stuffed animal, or a couple of pins, or various other small toys. If you want to go with a "per park" allowance, I am sure your girls could find something nice for that amount.
It all depends on your vacationing style, I guess.