Hey MamaLaura41
We see you're wanting to know if there is an option to change the setting for Audio Messages to never expire, and also prevent your child from changing it back.
You can change the settings for Audio Messages following the steps here: Send photo, video, or audio messages on your iPhone, iPad, or iPod touch
"The Messages app deletes audio messages after 2 minutes. To keep your audio messages longer, go to Settings > Messages. Scroll down and tap Expire under Audio Messages, then select After 2 Minutes or Never."
There is not currently a way to prevent your child from changing this setting back: Use parental controls on your child's iPhone, iPad, and iPod touch
If information above doesn't help, feel free to submit feedback and/or suggestions to Apple about this here: Product Feedback
Take care.