We had a similar but younger dilemma...
Our daughter turned 3 on a visit--however, on that weekend, we didn't do the parks--we did a return visit the following weekend and paid for her.
I am not a DVC member (YET), but when you reserve with CRO--they ALWAYS ask the ages of the children...withholding might save you bucks, but being honest about the age, might garner you some perks that might make your birhtday extra special. Personally--the savings isn't all that much (I think kids tix should cost a lot less than they do--considering height restrictions and such).
Also--keep in mind if you buy a Park hopper--perhaps, you may not use it all this visit, but on a return trip--it will make it more difficult to reuse since her birthday might be further behind.
It is a personal decision--hubby and I had AP for 6 months before our DD turned 3--but we opted to pay for her (got a Fla resident 4 day pass) when she did turn 3. CRO told us we were "grandfathered" for her and wouldn't have to--but when we double checked at the ticket window--they said no way! So if you get an affirmative in your favor--don't ask again

The answer might change.
On the ethical bandwagon--keep in mind the messages you are sending to your children by this action. Yes Disney makes lots of money--but if you are supposed to pay for something and don't--technically it is stealing.