I think I'm supposed to just need 400mcg, and I think the folate-sensitive people need 800.
My guess, right now, is that we should scale kids roughly based on how many calories they consume. My kids, ages 3 and 5, eat about half as many calories as me, so I'm going to aim for half as much as me. Doesn't address deficiencies in them, but part of me is really enjoying planning for getting most of this stuff from real food.
I started a little spreadsheet with folate per cup (since I am a wimp and don't think in grams)....
1 cup romaine lettuce (47g, 8 calories)... 64mcg folate
1 cup celery (101g, 16 calories) ... 36mcg folate
1 cup parsley (60g, 22 calories) ... 91mcg folate
1 cup spinach (30g, 7 calories) ... 58mcg folate
1 medium banana (118g, 105 calories) ... 24mcg folate (every little bit helps)
1 cup kale (67g, 33 calories) ... 19mcg folate
Plus there's some in things like broccoli and cauliflower, probably all the cruciferous veggies (among others), so I figure part from juicing/smoothie-ing and part from regular cooked veggies (need to double-check that nutritiondata.com reduces the folate from raw vs cooked, I like their data so far, but I'm not sure they're that nuanced).
And I'd consider, if you have a long-term deficiency, try to get 800mcg per day from food and then add on the Folapro in addition each day for a while, and then at some point decide you're caught up and just go back to food. YMMV, I've found that we have so many long-term deficiencies that I want/need both.