In Trinidadian and wider Caribbean English, to 'lime' means to hang out — to spend unstructured time with friends, usually outdoors, with music, food or drinks. 'We were liming on the beach' just means 'we were chilling on the beach'. The word has nothing to do with the fruit or the colour. It is a cultural staple in Trinidad and Tobago and is often used as a noun too ('a lime' = a casual gathering). If your child uses it they are likely connected to Caribbean culture or have picked it up from soca artists online. Nothing concerning — it is the Trinidadian equivalent of 'hanging out'.