I've had the same experience watching my (now 4yo son) voice search on my phone. It picks up what he's saying very reliably ... even when he's still laughing uncontrollably from the results of his last search :)

In case you were unaware, you can listen to past voice searches here: https://history.google.com/history/audio sometimes my wife and I will go back and listen to my son's searches for good laughs all around ;)