Transitive trusts and existing child domains

If i look at MS Technet article around AD Forest trusts it states:

"Each time you create a new domain in a forest, a two-way, transitive trust relationship is automatically created between the new domain and its parent domain. If child domains are added to the new domain, the trust path flows upward through the domain hierarchy extending the initial trust path created between the new domain and its parent domain."

Does this mean that this automatic relationship is not created for existing child domains that were there before the trust was created? Do these need to be done manually?

Reason for question is that I am seeing this where Root Forest domain for Forest A doesn't trust child domains of Forest B and vice versa.