For hyoscine butylbromide (Buscopan) injection, one product reference for infants/young children gives 0.3 to 0.6 mg/kg per dose, given slowly IV, IM, or SC in severe cases.

For a 26 kg, 8-year-old child:

However, pediatric injectable Buscopan labeling differs by country. The UK product information says the ampoules are not recommended in children, while New Zealand information provides the above weight-based dose. It should therefore be prescribed/administered by a clinician who has assessed the cause of the abdominal pain and screened for contraindications.

Do not use it if bowel obstruction/ileus, megacolon, myasthenia gravis, or suspected acute angle-closure glaucoma are concerns. Urgent review is needed for severe or worsening abdominal pain, guarding, bilious vomiting, blood in stool, fever, or marked lethargy. The New Zealand product information gives the pediatric weight-based range; the UK Buscopan injection SmPC states pediatric use is not recommended.

Do you mean the vial says 0.5 mg/mL? That would be an unusual concentration for Buscopan (hyoscine butylbromide). Standard injectable Buscopan is typically 20 mg in 1 mL, as confirmed in product information.

If it truly is 0.5 mg/mL, then the previously stated weight-based range for 26 kg:

That volume is impractically large, suggesting the concentration or drug may have been misread. Please do not inject it until the ampoule label is checked by a doctor/pharmacist.