Yes, there is a way to limit what can be played on specific HomePods in your house using the "Restrictions" feature in the Home app. You can set up "Content Restrictions" on each HomePod Mini to control what type of music can be played on them.

To set up content restrictions on a HomePod Mini:

1. Open the Home app on your iOS device.
2. Find the HomePod Mini you want to set restrictions on and long-press on it.
3. Tap on “Details” and then scroll down to the “Restrictions” section.
4. Tap on “Content” and select the level of restriction you want to apply (e.g., clean, explicit, or no restrictions).
5. Repeat this process for each HomePod Mini you want to set up content restrictions on.

By setting up content restrictions, you can control the type of music that can be played on each HomePod Mini, regardless of which Apple ID is used to play music on them. This means that even if your older child tries to play explicit content on their HomePod Mini, it will not play if the content restriction is set to "clean" or "no restrictions."

Note that content restrictions will only apply to the HomePod Mini itself, so if your child tries to play explicit content on another HomePod or device, it will still play if there are no content restrictions set up. Also, keep in mind that content restrictions only apply to music, and not to other types of content like podcasts or audiobooks.