different data SIMs on Watch and iPhone

https://www.apple.com/newsroom/2020/09/apple-extends-the-apple-watch-experience-to-the-entire-family/

From the fine print at the end of the above article:

• Family Setup will be available on Wednesday, September 16, as a free software update and requires cellular models of Apple Watch Series 4 or later or Apple Watch SE running watchOS 7, paired with iPhone 6s or later running iOS 14.

• Not all features will be available if Apple Watch is set up through Family Setup. A wireless service plan is required for cellular service. Contact your service provider for more details. Check apple.com/watch/cellular for participating wireless carriers and eligibility.

• To set up an Apple Watch with Family Setup, a new assistant guides users through the configuration steps on iPhone. Family Setup requires Family Sharing and an Apple ID for the child or older family member of the household, and cellular service enables all of its features.

• Family Setup is subject to local carrier support and will be available at 18 carriers in 12 countries and regions around the world at launch.

• Apple Watch will support 14 hours of daily usage with Family Setup.3

The https://www.apple.com/watch/cellular/ is the list Apple maintains on what carriers support the Apple Watch.

Before watchOS 7, I would have said NO.

I think it will still be NO.

Typical. The Apple Watch shares the iPhone's cellular plan and phone number. For this services the cellular carrier adds a monthly fee to the iPhone bill. In the U.S. the fee if $10 U.S. per month.

What I do not know about watchOS 7, is that there is now an option to have a parent manage a childs semi-independent Apple Watch. But I think the cellular carrier still needs to actually support the Apple Watch, and in this setup, you pay for a new phone number, and pay either a family plan prices for the extra number, or full prices for a new number. Your free eSIM would most likely not apply.

Also, many times iPad cellular services are data only, and do not include the other voice phone features, such as SMS/MMS and other cellular signaling technology.