I think it's fine to celebrate their birthdays when you go.
We're doing this ourselves. My twins turn 6 on Sept 3rd. Because of their birthdate, we've had to cancel 2 bday parties due to hurricanes (we were evacuated for Katrina and for Gustav/Ike). They've been so sweet, and never complained about it at all....but I think they deserve a Disney bday celebration after all of that! So, this year we are going to WDW in OCT (during fall break from school), as a surprise for the boys, and we'll celebrate their bdays while there! We'll do something small at home in SEPT - hopefully, there will be no hurricanes to ruin anything this year.