You could first of all increase the magnesium intake via diet
https://www.babycenter.com/toddler/feeding/magnesium-in-your-childs-diet_10324694

When an additional supplement is needed for medical reasons then I would certainly talk with your child’s GP or Peadiatrician

Also, after seeking GP advice always shop around for the best prices.

Best to see GP support – they probably have the best knowledge base.

I would ask my GP about this, especially before giving supplements to kids.

Best to ask a GP or in the pharmacy.

I’d probably ask a dr. I’d want to make sure I’m getting exactly what my child needs

Can’t provide any recommendations but a Pharmacist would be able to recommend you something suitable.