If a child took adults vitamin d gummies for more than 3 weeks what should i do?

Stop: giving them to the child. There is nothing to do at this point, it is unlikely there will be any effect to the child.

Vitamin D: Vitamin D is a fat soluble vitamin, so if the dose of the adult gummies was very high, the child would retain an excess amount of Vit D for a while. Hypervitaminosis D is a potential risk of excess Vit D intake, but is unlikely in this case. Recs for Vit D supplementation keep changing, but you may find this article helpful: www.mayoclinic.org/drugs-supplements/vitamin-d/dosing/hrb-2006040

Consider Vit D3 conc: by blood sample, optimal in the ~70-80 ng/ml range. This would clarify where things are at, otherwise just non-nonsensical guesswork/worry, likely of no value. Most vit. D supplements are quite low absorbed dosage/effect. The principal problem I have seen for >decade is very low efficacy of the vast majority of brands. Thus likely no significant harmful effects.

Contact doctor: Children who get too much vitamin D may develop abnormally high levels of calcium in the blood. This could affect digestion including constipation, vomiting and decreased appetite. Other signs include irritability, muscle aches, and fatigue. High blood calcium can also contribute to kidney stones. Contact the child's doctor to discuss; a calcium blood test may be needed.