Slime should always be made while under the supervision of an adult and it is recommended that children should be of at least school age before making. Always wash hands before and after playing with slime, do not ingest it and discard immediately if it starts to change color or odor.
Also keep in mind that slime made with borax can cause irritation, since borax is a cleaning solution and not meant to be constantly handled. Slime created with borax should be done so under adult supervision and hands should be washed regularly. Consider trying alternative slime recipes that do not use borax, such as baking soda or contact lens solution, which only contains very small amounts of boric acid.