Build a crafting station on your island. under the Administration option on the crafting station you can deposit silver if you are the owner.

Deposit the silver and also make your son co-owner of the building.

As co-owner he can then claim the building (via the building menu) and then access the Admin tab. he can then withdraw the silver.

You can also or course stand next to him, use the trade option, and directly transfer the silver.

Bot zzz

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