Is it possible to get information about internal transactions from the Tezos node RPC interface? If so, what is the call? If not, what other options are there for getting internal transactions apart from using an indexer?
The internal operation results are included in the operation "receipts."
Until a block is pruned, you can retrieve them (for example) by querying for the block:
For pruned blocks, e.g. blocks before your snapshot's save point, the receipts are deleted. You need to either index the relevant information as you go, or run an archive node.