On our trip my DS is turning 10. I know the age of child in the beginning of the trip is used for the entire length of stay. But... if we go to MVMCP after the bday I assume I have to pay adult price?
I disagree with the PP. We went when my dd was turning three. We were told she would be considered the age she started with for the ENTIRE trip including anything we did. We did do MVMCP and I asked if we needed a ticket for her - CM and TA told me 'no'. I would think this would apply to your son, also.
If you buy the annual pass and the child turns 10 during the year period you are still allowed to keep using the pass until it expires so just buy the childs ticket because technically at the time of the purchase the child is still a child and not an adult.
The world won't end, if you choose to keep the child 9 yo for the trip.

If you buy the annual pass and the child turns 10 during the year period you are still allowed to keep using the pass until it expires so just buy the childs ticket because technically at the time of the purchase the child is still a child and not an adult.
so then what do you do if not on the DDP? start paying for adult meals at buffets and restaurants?
see, that's where it would get weird for me then. because then what about the DDP, so do you pay as an adult for the DDP and tickets knowing the child will be turning 10? or do you start using child credits for an adult?

it makes perfect sense in your take on the situation, but now you mention packages specifically when in reality, it can be as simple as just tickets or just the resort. disney doesn't make any age distinctions at all moving past the check-in point, that's my take on it. and i have yet to see anything in print to argue that.
so are you saying that if someone doesn't buy a package and buys just tickets only, that the ticket becomes invalidated once the child turns 10.
as to the original question, i'm inclined to side that you're whatever age you are when you first started your vacation. despite celebrating birthdays on whatever day you wish, you do not age at disney. the implication of suggesting that you need to purchase an adult ticket for a party because the child is turning 10 the day before or whatnot is that you would have to start paying for every meal once a child turns 3 on vacation. and then if you purchased the DDP but didn't buy it for a 2 year old, where does that leave you? unless you draw a distinction between the DDP and paying OOP.
your vacation doesn't 're-start' because you go to a party.