Looking for weight gain and immunity.

Hi, My kid is 7 year old. She will turn 8 soon. Her weight is 24.1 and height wise  she is bit tall. Coz her dad has also height . But she is too thin, she likes more of sweets, she has fruits, but vegetables very less, non veg she has chicken, egg. But still her weight is less, I feel. Looking for some multivitamin syrup or calcium to build her immunity. And how long should i give her. Please mention it here. As m working full day, don't get time to take her down also much to play. We both are working full time . Me n my husband. She is our only daughter. Kindly guide. Her physical activity is very less at home. Some good multivitamin or calcium or protein for her weight gain and good immunity. Elsewise she is fine, no other health issues.