Should an Apple Watch with eSIM be paired with SAME iPhone that has the same SIM number?

I am providing my young children older iPhones (SE 1st generation..) that will use the same SIM (phone number) as their eSIM Apple SE 2nd generation Apple watch. Since the iPhones have an old iOS vs. that of the Apple Watch, I am unable to pair the child's apple watch with their (old model) iPhone; instead I have paired the apple watches with my (newer) iPhone with also Family Setup.

Is there a reason why the multi SIM (i.e. eSIM and physical SIM) would not work independently? I had technical issues yesterday and called Apple Care... but not sure I received a clear answer. I will call again today, but wondering if the Apple Support Community has faced the same issue; thank you.