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The family plan is available for family members who live under the same roof. There should be no issue for you to create a Premium Family plan and add your kids to it.
Note that this should happen at either your home or that of your wife, as when you set up the plan, you need to enter a home address, which needs to be matched on each device when they accept the invitation to join the family plan. This should also happen when you're physically on that address, otherwise it won't work.
So, what you can do is set up the plan and the kids accounts the next time when they're staying with you. Or you can set up the plan when you're at your wife's. As long as the kids come to the original address from time to time everything should be OK. Note that the kids need to have their own compatible devices to use the Kids app.
You can read more about how Premium Family works - here.
Let us know if you have more questions.
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