Cellular Apple Watch won’t send/receive texts from android

If you can’t send or receive messages on your Apple Watch - Apple Support

See item #2

• Basically, your iPhone must be turned on.
• Your iPhone must be able to access the Internet (it does not need to be with you, just able to contact your Apple Watch via the internet
• You must be signed into iMessage on your iPhone and Apple Watch

Read the above link and make sure you follow the instructions for SMS access.

The Apple Watch radio ONLY does Internet data, and phone calls are Voice over IP calling.

SMS requires a traditional cellular radio. The Apple Watch, when paired to an iPhone just arranges for the iPhone to forward SMS messages between the iPhone and the Apple Watch, as the iPhone has a full featured traditional cellular radio.

On the Family Plan, SMS/MMS is not possible, as the parent's iPhone SMS/MMS messages would be associated with the parent's phone number, and not the dependent child's. If Apple allowed the child to use your iPhone's SMS everything would look like it came from you.

I do not have a lot of experience with 3rd party internet based messaging apps, and even less with what might be available on an Apple Watch. But if there is a messaging service that works on the Apple Watch and Android, then that is the way your child can message with an Android user.