attachedmama, I asked a lot about this when I was getting ready to tandem my first two. I wondered if there was a set/finite amount of colostrum, or would it be produced supply and demand as milk is.
I still don't have a good source of scientific info on this, but here is my logic:
The milk is going to come in at roughly the same time, regardless of whether you're nursing a toddler or not. It stands to reason that you will continue to produce colostrum right up until that point, so I don't see any reason to limit the toddler. It will be there for the newborn.
Of course, it's always a good idea to nurse the newborn on the opposite side from what the toddler just had... at least in the first few days.