I can't imagine a tail blade truly developing until after birth if, in fact, Andalites give live births (still unproven, btw).
As I (and others) have mentioned before, Andalites are NOT mammals! This is important to keep in mind! Mammal is an Earth classification of animals. Just b/c Andalites appear similar to mammals in many ways does not mean that they ARE mammals.
Another note, in regards to the carrying (post-partum) of young....I do not believe that this would be possible. Andalite infants, while likely small are still probably too large for weak Andalite arms to handle. Besides this, touching is generally avoided in the Andalite society. They are not intimate in the way that humans are. This could be a result of the lack of close physicality in infanthood.