It would be especially harder to dose accurately with liquid or large tablets. He apparently has personal preferences for the one product referred to in the article on the website. I would go up, gradually, to one to four tablets a day for a child, and to 2 to 8 tablets per day for an adult. If the hyperactivity continues, the child may be telling you, in effect, that he or she needs more folic acid.
Folic acid is a very safe B-vitamin.
When you start a trial of DMG, don't confuse the results by simultaneously starting other vitamins, drugs, or other forms of treatment that might make it difficult to tease out the effects of DMG from the effects of the other things tried. The DMG is merely a highly concentrated food. It may in fact improve the effectiveness of the B6. 125 mg tablets is for purity, dosing, convenience, and shelf-life issues. It would be especially harder to dose accurately with liquid or large tablets.