My Child is always sick in her First Years  

I'm a young mom and have a first child who almost 1 years. Me and my husband works from 09AM - 05PM so we do not have enough time with my child, she is taking care by the baby sitter. Once she is 4 months, she is fat and the weight almost 6 kgs and now once she is almost 1 years, the weight is only 7,5kgs, this is not normally weight for the baby ages like her. The problem is she always sick in almost on every months during her 5 months ago and do not much like to eat and drink the milk. It's make me so worries and confuse. Give me your suggestion, how to handle her and what's the good menus and what's the good milk ?