In the Code of Canon Law, where it deals with Participation in the Blessed Eucharist, it says:
Canon 914. It is primarily the duty of parents and of those who take their place, as it is the duty of the parish priest, to ensure that children who have reached the use of reason are properly prepared and, having made their sacramental confession, are nourished by this divine food as soon as possible. It is also the duty of the parish priest to see that children who have not reached the use of reason, or whom he has judged to be insufficiently disposed, do not come to holy communion.
So, it would appear from the above canon that First Confession should precede First Communion.