Hi Jennifer,
The discounted price of $10 is for the first camp within a family. In other words, if you want to upgrade 2 kids, one subscription will be $10 for the first month and the other subscription will be $39. Every subsequent camp will be the normal $39/camper/month.
To your second point - you're more than welcome to downgrade a member's Camp subscription at any point. The member will be able to access the Camp until the end of the 4 week session, at which point their access to the Camps app will be closed off and you won't be charged moving forward. You can re-upgrade the member's account at any point in the future.
Hope that explains everything for you - let me know if you have any lingering questions!
- Brian