What medicine can we give to the child age 3 yrs old if she has a stomach ache?

Depends. On the cause of the stomach ache. If it is constipation then p fruits can help (peach, pear, plum, prune, pineapple, pumpkin, peas, papaya, and pomegranate). If it is too much rich food homeopathic pulsatilla can help. If it is a virus, then homeopathic nix (permethrin) vomica can help. More info is helpful to determine best answer.