Margaret,
What fun ages for a Disney vacation! You will have a great time.
I think the answer to your question hinges on what the trinkets might be, as those prices can really vary. I think instead of giving you a dollar amount, I might best be able to answer your question by giving you a few sample prices of popular souvenirs (prices are approximate and current as of 10/09). You can decide from there what your girls might like and establish a budget.
classic Mouse ears - $10 and up
t-shirts - $20 and up
stuffed animals - $15 and up
trading pins - $9 and up
Disney Crocs - around $35
charm bracelet - around $10
charms - $4 and up
figure sets - $12 and up
Those are a few of my daughters' favorites.
If you are looking for ways to save money on souvenirs, I strongly suggest visiting your local Disney Store or
Disney Store online to purchase t-shirts, stuffed animals, and other Disney merchandise at a reduced cost. I also like to stop by the dollar store and buy glow sticks for the kids to play with at night. I pay $1/dozen and it keeps the girls from asking for expensive light-up toys in the parks at night (and believe me, they are everywhere!) Another tip I have is to bring along some kid makeup and purchase a tiara or hairpiece in Downtown Disney. You can give your girls a mini-makeover for a fraction of the price of a Bibbidi Bobbidi Boutique visit!
Have a wonderful vacation!