Depending on their age, I don't think it's a problem to let them sip it, but it's formulated for adults and not for kids.
Our pediatrician still has our kids on 1/2 flintstone vitamin/day at 2 and almost 4. Their bodies have different metabolisms and need different amounts of vitamins vs. ours.
I haven't really looked at the label, so I'm sure it's not harmful, but I can't find a good picture of their label online to really be able to see what the different vitamins/quantities are.
I'd simply ask your pediatrician if it's something you're wanting to do just to make sure that any of the vitamin components in there aren't harmful in their current quantities.