Is it permissible for the male and female children of paternal uncles to play together?

If male and female cousins are young and have not reached the age of sexual desire, and there is no fear of temptation, then there is no harm in them playing together. However, if they are older, it is forbidden for them to do so, because cousins are not mahrams; rather, they are considered non-mahram (strangers) to their female cousins, and the same applies to maternal cousins.