in "horse around", "horse" is used as a verb, "to horse around" means "to play or joke with someone". Those two kids were just playing, they were horsing around.
to horse around意思是"和别人开玩笑、闹着玩儿"
别人开玩笑或者是恶作剧，也可以算是horsing around
To jump the gun is to do something too soon. For instance, if a runner starts running before the race starts, he's jumping the gun.
to jump the gun就是过早采取行动的意思,操之过急
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