Hi Shelly!
Is it possible that it can ever be too early for cake?! :) You didn't mention what character breakfast you are attending. I would first recommend you have your daughter wear some type of apparel that would compliment the characters you are dining with. For example, when my daughter was younger, she would wear a piglet t-shirt and piglet ears (headband) when dining at Crystal Palace. This always got Piglet's attention! You may also want to consider picking up a birthday button at any Guest Services location.
As far as your cake/no cake question, you may want to consider a mini surprise cake that can be found
here (there are also a ton of other celebration ideas located on this page). The chest can be closed and you can save the cake for later. If you have private dining at your resort, you may want to also consider having a cake delivered to the room for later in the day.
As far as your disney dollar idea, I think that is also a great option! Kids love to have money to spend and having money with disney characters is extra special.
I hope your daughter's birthday is magical! Diane