I tried to persisit a tree structure using UDT and mapper, but got a StackOverflowError. The problem is the endless recursion on UDT mapper creation for the circular referenced class member.
java driver 2.1.X and 2.2.X
I don't think UDTs can be recursive. You've found a workaround with ALTER TYPE, but notice that you can't insert recursive data, the second level doesn't have the children property:
So I'm leaning towards ALTER being too permissive. I'm checking with the Cassandra devs, if that's the case I'll open a ticket in their JIRA to prevent that kind of schema to be created.