The short answer
Do not scale an adult dose by weight. Children are not small adults in the ways that matter here: their absorption differs, their upper intake levels are lower, and some compounds have pediatric specific considerations that have nothing to do with size.
Use a product formulated and labeled for the child's age, read the age band on the label rather than the weight, and treat the tolerable upper intake level as a ceiling rather than a target.
Why you cannot halve a capsule
A split capsule does not reliably deliver half the dose, because the powder inside is not uniformly distributed and some of it is lost. More importantly, several capsule types are designed specifically so that splitting does not work: enteric coated capsules resist stomach acid, and time release formulations depend on the intact shell.
Then there is the arithmetic problem. Adult doses are set for a 70 kilogram adult, and a 20 kilogram child is not a third of an adult metabolically. Children have higher requirements per kilogram for some nutrients because they are growing, and lower tolerance for others because their clearance pathways are still maturing.
The upper limits are lower than you think
The tolerable upper intake levels for children are substantially below adult values, and the gap is wider than the weight ratio would suggest.
Zinc is 40 mg per day for adults and ranges from 7 mg for toddlers to 34 mg for teens. Vitamin A is 3000 micrograms for adults and ranges from 600 to 2800 depending on age, and it is the fat soluble one where accumulation is the concern. Vitamin D is 100 micrograms for adults and 63 to 100 depending on age, with lower values for infants. Iron is 45 mg for adults and 40 mg for children under 13, which is a smaller gap than most people expect, and it is the one where accidental overdose does real harm.
The practical consequence is that a child taking an adult multivitamin plus a children's gummy plus a fortified cereal can cross a ceiling without anyone noticing. Add them up once.
Iron, the one that sends children to the hospital
Iron is the leading cause of fatal poisoning in young children in the United States, and it is almost always accidental access to an adult supplement. This is not a theoretical warning.
The mechanism is corrosive injury to the gut lining followed by systemic toxicity, and the dose that causes it is not far above the therapeutic range for a small child. Gummies are not safer, because a child who finds a bottle of iron gummies will eat it like candy.
Keep every iron bottle, adult or children's, somewhere a child cannot reach, and treat the child resistant cap as a convenience rather than as the safety system.
Which forms work for which ages
Liquids and drops for toddlers and preschoolers, because they cannot swallow pills and because a dropper is the only way to measure a genuinely small dose.
Gummies and chewables from around age four, with the caveat that gummies are a choking hazard for younger children and that they contain sugar that sits on teeth.
Capsules and tablets generally from around age eight to ten, and only when the child can actually swallow one reliably. A child who gags on a capsule will not become a consistent supplement user, and the format that gets finished beats the format with the best spec sheet.
Our kids magnesium drops are drops for exactly this reason, and the appetite gummies and choline gummies for kids and teens are gummies because the age bands on those labels start where gummies are appropriate.
Read the age band, not the weight
Pediatric labels give an age range and a dose for that range, and that is the number to follow. Weight based dosing is what clinicians calculate, and it uses different reference data than a consumer label does.
If your child is at the edge of an age band, use the lower band. There is no prize for the higher dose, and the ceiling is the thing you are trying to stay under.
The two questions to ask before starting anything
Has anyone actually identified a deficiency or a reason? Supplements for children are most defensible when there is a diagnosis, a dietary pattern that explains a gap, or a recommendation from the pediatrician. Supplementing a child who eats normally because it seems like it should help is the least defensible version.
Does it interact with anything the child is already taking? A children's multivitamin plus a children's gummy plus a fortified breakfast is the most common source of accidental double dosing, and it is worth writing down everything for a week before adding anything.
How to count drops properly
A dropper is not a precise instrument. Drop volume varies with the angle you hold it, with the temperature of the liquid, and with how hard you squeeze the bulb. A "full dropper" from one brand is not the same as a full dropper from another.
Count drops when the dose matters, and use the milligram line on the label rather than the drop count anyone suggests online. If the label gives both a drop count and a milligram amount, the milligram amount is the real one.
What to do if a child takes too much
Call poison control, which in the United States is 1-800-222-1222, and do it immediately rather than after you have looked symptoms up. Have the bottle in your hand when you call, because they will want the ingredient list and the strength.
Do not induce vomiting, and do not wait for symptoms. The poisons centers handle these calls constantly, and they will tell you whether it is a watch at home situation or an emergency department one.
How much magnesium does a child need
The RDA ranges from about 80 mg per day for toddlers to 240 mg for teenage boys, and 360 mg for teenage girls. Supplemental magnesium above the age appropriate level causes the same osmotic diarrhea it causes in adults, and children notice it sooner.
Can my child take my gummies
No. Adult gummies are dosed for adults, the upper limits are different, and the sugar and sweetener load is not scaled either. Use a product with an age band on the label.
Are children's supplements regulated differently
They are regulated the same way, which is to say not premarket approved. What differs is the labeling requirement for age bands and the practical expectation that manufacturers set lower doses. That is a market convention, not a guarantee, so read the label.
Should I give a daily multivitamin
Most pediatric guidance says a child eating a varied diet does not need one, and that a multivitamin is reasonable picky eaters, restricted diets, and specific diagnosed gaps. A multivitamin is not insurance against a bad diet, and it does not replace food variety.