I was thinking about it last night, and I think I will simply do a "rule by 3" :
1/2 teaspoon soda (2,5 gr) / (my weight) lbs x (?) soda / (children's weight) lbs
That would be about 0.5 gr Soda for a 30 lbs toddler
I'm going to look up if it's better to give it in water alone, or if it could work in orange juice (vit C), similar to the Pedialite Recipe (orange juice + salt diluted in water)