Hurting because of memory alignment or something else?
Given we have indices along the serialisation for child nodes I would have thought not having nodes start at the consistent intervals doesn't hurt us for traversal at least.
I'd quite like to use this idea for a file backed immutable tree. The array of children would have variable size with bit field to index then nth child in the sparse array.
Constant time array indexing depends on a fixed array member size. So, the actual memory layout of storing the nodes with non-fixed size could not use an array of nodes.
I suppose you could (should) write your own allocator to ensure nodes are allocated in a contiguous piece of memory, and have a separate array of indexes / pointers that tells you where the n-th node starts. That is another array that'll have to be kept in cache though.
