Your dependents' user profiles should already be loaded under your account and all you need to do is sync the brush.
To set up the brush, have your dependent log in under your account on the Beam Dental app. Tap or swipe the > next to your profile picture until you get to their profile. Scroll down and click "Add Brush" or "Swap Brush." Then complete the brush syncing process to link up the brush to the account. Done!