According to 16 CFR Part 1700 (1700.20 ii), for unit dose packages, a failure is “the number of individual units which constitute the amount that may produce serious personal injury or serious illness”. This is based on a 25lb (11.4 kg) child. Keep in mind that toxicology is just one method of harm. Eye or skin irritation, flammability or other risks should also be considered.
Each company needs to decide what constitutes “serious personal injury or serious illness” for their product. For highly toxic or harmful products, the failure level is usually set at one (f = 1); access to one unit is considered a failure. For less toxic or harmful products, the failure level may be higher; a child could open some limited number of units.
The US test sets a default limit of “greater than eight”. This means that, unless a lower failure limit is declared, a failure occurs when a child gains access to a ninth unit.