I think the reduced sugar OJs have artificial sweeteners in them, so it wouldn't be my first choice to give that to a child. (I can taste artificial sweeteners, so I don't consume them at all. DH thinks I'm weird, but he's not the one tasting the chemical bitterness!) As for brands, well, I find that Tropicana is consistently sweet while other brands can be bitter in the wintertime. Be sure to shake the container before you pour a glass - I've heard that the calcium can settle at the bottom of the container.
Calcium chews may also be an option, but you'd have to check with your pediatrician - too much calcium in supplement form can be constipating.
If all else fails, calcium & vitamin D enriched non-dairy "milk" such as soy, almond, coconut, and rice milks are certainly an option. They come in flavored/unflavored as well as sweetened/unsweetened versions. But you do need to read the labels because their calcium contents vary based on brand & "milk" type.
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle