Macbeth = shortest of the big 4 tragedies
R & J = shortest early play
Comedy of Errors = shortest over all I think
NOT Lear or indeed any play dealing with trauma and madness, janh.
Would also not do Much Ado, because although it's my favourite comedy I think it's too complex and poignant for youngsters.
I would consider 12th Night, Midsummer Night's Dream, Henry V for a child interested in history (good battle stuff etc) and maybe As You Like It. Also the Tempest for monster and shipwreck factor.