If it was me, I would probably try to bring $500.00. This is how I would break it down...
$100 - get Disney Dollars in different denominations for the kids to have as their money ($50 each). This is money that they can spend on whatever they want, but when it is gone, it is gone.
$100 - for you to use for bigger souveniers for the kids - plan on getting them each a t-shirt, mouse ears with their names embroidered on them).
$100 - to use for souveniers for yourself
$100 - to use for souveniers for others at home that you will want to bring back something special for (MIL, etc)
$100 - to pay for any additional snacks or other incidentals