Yes, if your family members accepted the invites to join your Premium Family plan, they won't be charged for their accounts any further. Their payments are now handled by you as the plan manager, and they’ll be added to your Premium Family plan when their current billing periods run out.
You can read more about frequently asked questions regarding the Premium Family plan in this Q&A thread.
Let me know if you have any other questions, and have a good day!
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Hi what will happen is wait until your two children wait until their Spotify premium runs out, once their Spotify premium runs out then you can add them into your Spotify Family plan hopefully this helps you thanks.