sim in apple watch instead of iphone

There is no physical SIM in the Watch. The watch has an eSim.

If her cellular service provider supports the Watch, then you can buy a GPS+Cellular SE watch and add it to the iPhone's service plan. It's usually $10 to $15 a month more. The watch can make and receive calls independent of the iPhone.

You can't pair an Watch with an iPad.

Although the watch is water resistant, shampoos and other soaps are bad for the watch. They can eat away the water-resistant seals and then allow the water to ruin the watch.

M1 Mac mini, macOS 14.3.1;  Watch, series 7 GPS/cellular, watchOS 10.3.1; iPhone 12 Pro, iOS 17.3.1; TV 4th Gen, tvOS 17.3; HomePods, audioOS 17.3 and Airport Express w/ Airplay 2

No, what you are proposing isn't possible.

An Watch is an iPhone accessory, not the other way around. Except an Watch set up for a family member without an iPhone, the iPhone carries the primary service from a cellular service provider. You can then only add service for an Watch to the iPhone's account if the cellular service provider supports Watches. It isn't two accounts, it's one account for the phone that supports the watch as an add-on.

I doubt that such a supermarket service provider as you describe supports Watches.

depending on the carrier then one can have the same number and contract on both a watch and phone

if the watch does not have a esim set then it will work like if it did and receive calls and the likes but only when the phone is within bluetooth range of the watch.

if the watch is not within reach and it still has to work for calls, and sms then the only ones you can get help from is from the carrier

Dah•veed wrote:

I doubt that such a supermarket service provider as you describe supports Watches.

If we are talking Tesco Mobile or the like then there’s no chance.

Apple Watch - Network Providers - Apple

Two more considerations, so long as the iPhone is within Bluetooth range she can make and receive calls on the Watch without needing a cellular model. Also fall detection is about 50% effective, however she can press the SOS button if the worst happens.

If I am left alone in future I might risk the soap thing for the sake of safety.