Start with one keeper out near the touchline and the other in goal. The one out wide floats a ball in for the keeper in goal to come out and take the cross. Work on perhaps ten or so good deliveries and swap goalkeepers. When a similar amount of good balls have been delivered in change goalkeepers again and cross from the other side. With juniors the balls could be thrown in and moving up to keepers at a higher skill level of course struck in with the pace and ball flight being changed when everyone has had a go. You can have any number of participants in a crossing drill, for instance alternating crosses with a person in front putting pressure on the keeper when he or she comes for the ball. Maybe one or two near the halfway line so if it is a clean take you can add a quick release and simulate starting an attack. There are endless variations just waiting for you to put your mark on them