Swallowing button batteries can seriously hurt or kill a child, even ones that no longer work.
Make sure you store them in secure place in your home and be mindful if children are playing with items that contain them. Not just small toys but things like calculators, car keys and scales.
If you think your child has swallowed magnets take them straight to A&E. They may have stomach pain, vomit, have a fever, or point to their throat or stomach.