Starfish, it's fine to submit additional pieces to Boys' Quest, Hopscotch, or Fun for Kidz. Their 'official' guidelines sheet they enclose with rejections says please not to send more than one submission per week.
As for submitting additional pieces to other publications, if they take six months or more to respond, and especially if they send their submissions out to first readers, I've occasionally sent a second ms to them before I've heard back on the first.
If you send multiple pieces close together that are the same genre to mags that are unthemed, you do run the risk of competing against yourself.
Best wishes to you!