But to be fair, there is a distinction here. That is, if you're not the registered Alexa user and you're not the one who agreed to the usage terms, shouldn't Amazon try to recognise that your voice is different and not store your voice commands? It's clear they have the recognition tech to do this. I'd say it's a pretty reasonable expectation.
Even if you posit that consent is implicit in using Alexa (i.e. if you use a friend's, for example, in making that choice you're consenting to Amazon storing your voice command) - something that's debatable but certainly within reason - I'd say you definitely can't extend this to children, who simply can't be expected to even know what this means let alone understand the implications.