Just to try to be helpful and clarify.....they need to qualify at a show in a YOUTH designated class with the horse (youth halter, youth jumper, youth obstacle, youth driving, etc.) they are then qualified to show in any youth class at nationals.
Putting a child in an open class -- like a color class -- does NOT count in qualifying them to show at nationals in a youth class with that horse. There has been, and I think still is, confusion about that and we've been at shows where this has been questioned -- usually when someone wants to put a child in color class with a horse because they think then that child is qualified for nationals to show that horse in youth classes.
We have discussed this with the AMHR office and asked for the youth rules to be clarified to be sure.
We personally have five kids that show and our show string ranges from 8-14 horses and we are very careful to make sure every child of ours is properly qualified with any horse they may be showing at nationals that year. Thankfully my husband keeps track of all that on his computer because with our family and horses I'd be overwhelmed in no time trying to track it all.
FYI -- I have heard that are checking now to make sure youth are actually properly qualified too, because of confusion in the past about it.