Warnings

The site Neymours KidsHealth says, toys should be large enough — at least 1¼ inches (3 centimeters) in diameter and 2¼ inches (6 centimeters) in length — so that they can't be swallowed or lodged in the windpipe. A small-parts tester, or choke tube, can determine if a toy is too small. The base model Magnagon is slightly smaller, so use a bigger design around kids less than 11 years old.

Also, a New York Times article says that the magnetic balls used in the 3mm Magnagon caused 7 deaths and multiple surgeries after kids with an average age of about 7 years old. This warning is meant to caution you to not bring these magnetic balls near smaller children. But there are some magnetic balls that don't have this problem, like 2.5mm Speks.