To answer your question directly – no, there’s no voice security enabled on the Echo to prevent this from happening.
However, I would suggest using 2 separate Amazon accounts as part of a household – and only having one of the Amazon accounts be linked to the heating skill. This way, if the child says “Alexa change the temperature” to their device, there won’t be a skill to enable it.
To set this up, make sure the child has their own Amazon account, and do not set up the heating skill on it.
Then, ask the child’s Echo “who am I?”. Alexa will respond with who’s account she is currently in. That will help you confirm that the child’s account is the active account on the device.
This isn’t foolproof though – the child could always say “switch accounts” and switch to the account that has the skill enabled.
Hopefully that helps you accomplish what you’re trying to do!