Allowing arbitrary number of children doesn't come for free. You might need a linked list, which is a waste of memory and access time if you always just use up

Allowing arbitrary number of children doesn't come for free. You might need a linked list, which is a waste of memory and access time if you always just use up to two elements. What most parsers do is to provide exactly 2 slots, aptly named left and right, and use zero, one or both of them.