ensure the child device is powered off prior to adding or removing limits.
From the Smart Family parent app, select the appropriate family member from the top row of contacts (multiple may appear).
From the Manage controls section, tap Limits.
Tap Data target.
Enter the data limit in GB (Gigabytes).
Once the limit is reached, data usage is restricted until the next billing cycle.
You and your child receive free text alerts when they've used 50%, 75% and 90% of the limit.
If desired, select Don't block data, but alert me when the limit is reached to turn on or off.
On when a check mark is present.