My MIL always gave each kid $100 for spending money each trip. We would help them budget $25 per park, with any unused being rolled over. We would let them look, but also promised to come back to get anything towards the end of the week, so they had time to think about a purchase.
For the most part, my kids have made very considered purchases. My younger DD loves pandas, so a stuffed panda from China was very loved and appreciated. My youngest was obsessed with Perry a few years back, he bought a pillow pet that he cherished. Funny story--my older son, now 19, saw the Pokemon cards in Japan, and insisted on spending all his money on them. I reminded him of the $25 per park plan, and that he could buy such cards at the local Target when he got home. No go--he wanted them he wanted them now. He spent every dime he had on those stupid cards. And within two days, regretted his purchase, because he had no money left for Tatooine Traders or the Pirates store. But, he was ~13 at the time--not a little kid--and he doesn't learn a lesson unless it's painful. And honestly, if he'd been content with the cards, I would have been fine, but I knew darn well he was going to be kicking himself later.
As to the cousin, I would prep your child ahead of time that, "this is Sarah's only trip, she'll have more spending money", so your child understands. If you go frequesntly, your kid doesn't need a Space Mountain t-shirt every trip, but it might make a nice memory for her cousin.