For a 2 year old, I wouldn't budget a thing. I would just use any 'extra' cash to buy one or two things if the child sees it in the store and falls in love with it (and it is inexpensive). OR, even better - buy one or two Disney items from Big Lots before you go and present them to the child down there - just as good as buying them there but a lot cheaper.
Seriously, for a 2 year old I wouldn't worry about it. He/She won't know the difference. AND, remember - what you do now will dictate years to come - so if you want to keep # and $$ of souveniers low for the future - START NOW!