In the books Treebeard laments the ents' loss of their entwives -which is why they can't have children anymore. However the way Treebeard describes the entwives, it seems that they (ents and entwives ) spent some time together before they parted. Now when they spent time together they must have had children - if not how can there be so many ents?
Treebeard is said to be the oldest ent. If Treebeard is like Adam, then all ents are descended from him, like how all men are descended from Adam! However this might not be completely true because
Treebeard is the oldest ent alive
This suggests that Treebeard might not be the first ent. But in that case, if Treebeard is son of the oldest ent and former ent leader (which is why he is the leader of Ents) he must have had children! Adam isn't the direct father of all of us! Adam's sons can be considered as our fathers because we are descended from them! Similarly for Treebeard.
So did Treebeard have any (ent) children ?
Interesting note : The female ents are called 'Entwives'. Now a wife is a woman who is married to a man. This suggests that Entwives might have been married to Ents. Though this simply might be a blooper.