if you will usually be travelling summers, perhaps you could consider an April Use Year, if there are any left. (April UY has greatest flexibility for trips falling in Apr, May, Jun, Jul, Aug, Sep)
w/ an April UY, you could do Magical Beginnings to sell back your 4/2003 points at $10/pt and apply that amount to your down payment.
then when you get your new points in 4/2004, you can bank those into 2005 and can then use double your points during your 2005 Use Year (4/1/2005 - 3/31/2006).
so you can get a "discount" w/ MB, AND you'll still start off banking.
anyway, it's pretty hard to actually know for sure what your vacation patterns will be over the next 39 years though.
so, take any use year picking advice with a grain of salt.

(i.e. Dec UY or Apr UY could both end up working just as well for your family in the long run)