All devices that are registered to your account will have access to account information - orders, payment method, content, etc.
If the kids are young, you can use FreeTime child profiles on their tablets to limit what they can view and access.
Otherwise, open a separate Amazon account and use that for gift purchases. If you have a Prime membership on the current account, you can share some benefits (such as Prime shipping) with the new account by linking the two accounts in an Amazon Household.
You can link two accounts in an Amazon Household. You can also add teen/child profiles, but these are NOT separate accounts. The child profiles can only be accessed through FreeTime, and the teen logins are very limited. See these help pages - About Amazon Households and Share Your Amazon Prime Benefits.