Handball attack games and physical training, U10-U12, Denmark 

At this age it's about building a base of generic movement skills and efficiency. Once the pupils have this base the conditioning (i.e. the running you mention) can come. If kids are inefficient then what is perceived as a lack of conditioning could simply be that the kids don't have the strength and coordination within their own bodies to demonstrate their conditioning. Given the stats on youth sports and how many children actually make it to professional athletes it makes sense to keep their movement options open. Once they've got a general base of movement skills, specialising later in their lives will mean they have a good understanding of how to move their bodies and will make the handball specific coaching (and conditioning for that matter) much easier.