I have a 4 and 5 year old. Honestly, I don't think they really understand money yet. I love budgeting and I try to teach them all the time what a penny, nickel, dime, and quarter are worth, and that they add up to dollars. I take them to the bank and give them all the money we find around town. They know what money is, and they know to use coupons and look for sales. We talk about how much things cost. But, they are too young to fully understand the entire concept beyond they can't get X if it's over X amount.
I think it will be easier on you to limit the souvies to 1 big ($20+) and 2 small (less than $10), or something like that. I've done that and it's worked well. Getting down to dollars and cents seems a little hard to me. And, I'd limit it to no more than $50 for that age. Just my honest opinion.