Child resistant packaging has been defined as meeting the Consumer Product Safety Commission test protocol. Design of these packages starts wtih understanding the materials, function, costs and user acceptance. The nature of the test is subjective, because actual children and adults are used for this testing. My suggestion is to start out by working with an ASTM certified testing lab with a lot of experience testing child resistant packaging according to CPSC guidelines. These professionals can give you good advice and guidance on your package before testing. Usually, the best approach is to build multiple steps into the opening feature that a child of the test age does not have the cognitive skills to figure out quickly. The other design feature is to consider the size and strength of the childs hand vs the adult hand and use that information to direct your design. Protect your investment with a patent attorney to make sure you are not using an opening feature that is already patented by someone else. Testing is expensive, so you want to make sure your design will pass the first time. This is not always possible, and you might have to modify and retest for the best possible outcome.