What benmc said... My biggest thought is protect the keepers.
My experience with U13 & below here in Northern New England in the US (though I suspect it's universal(?), is that coaches focus way more on training their field players than they do on training their keepers. The result is a lot of kids in goal who genuinely don't know how to keep themselves safe when making a save.
The Laws quite liberally define possession (ball between hand and any surface, touching it with any part of the hand or arms, etc.) The Laws also don't specify the duration of possession.
At this level, I make it clear to coaches before the game that I'll be blowing the play dead if I see a hint of possession, in order to keep their kids' dental work intact. All except the most testosterone-poisoned appreciate this.