R.G.
K.,
Your new baby will absolutely have enough colostrum. Your body begins the change over at around 4-5 months during the pregnancy. This is automatic based on the pregnancy itself and is not affected by the nursing of your toddler. Some women find that their nursing children will naturally wean during this time because of the change in the milk, or the amount produced.
Here's a great page that answers your question very well. http://www.babycentre.co.uk/baby/breastfeeding/breastfeed...
And another
http://www.myntoddler.com/comments/index.shtml?cat=preg/p...
Blessings to you on your tandem nursing journey.
Rolinda
Wife, Mother, Friend