Unfortunately, the Rinkeby test network’s Etherscan doesn’t appear to track internal transactions. If you compare the testnet version of Etherscan to the production version of Etherscan, you see different menus — notably the ones related to tracking internal transactions.
I’ve attached comparisons between your contract on the testnet, and the interface of a random contract on the mainnet. As you can see, Etherscan on Rinkeby clearly doesn’t show the same menus and the same options. Unfortunately.
Since internal transactions aren’t tracked by the blockchain by default — you have to actually tally each one as it goes off — it’s possible that Etherscan simply didn’t start tracking them early enough and now tracking them would result in inconsistent data.