Don't be so sure that they carefully research their articles to make sure everything is consistent.
They have no central direction and no "oracle" any more. Their doctrines are a mishmash of the less wacky of Fred Franz's ideas and various random reinterpretations by Splane & company.
Best bet? Someone in the writing department just used "children of God" because it sounded nice & warm & fuzzy and his proofreader didn't think about "official" WT doctrine about not becoming "children of God until after the 1000 years" because (1) in WT-world, it's not a particularly important point, and (2) as noted above, outside of the core doctrines, it's such a tangled mess who can possibly keep track of it all?