I would think 9 balls of 50 g would be fine for most baby dresses but it really depends on several things.
Long sleeves, short sleeves, no sleeves? Long dress, short dress? Full skirt, straight skirt, a line skirt? If it were long sleeved, AND had a really gathered skirt, AND was a long dress, then you might start to run into problems.
To give you an idea... here is a dress from GARN studios free patterns. It's a longish dress with no sleeves. Their 6-9 month size, which seems to match the chest size you are talking about, uses 300 grams (and that's because it has two yarns.. might be a little less if it were all one yarn).
This dress is done on 4.5mm needles and looser gauge.. so it will use less yarn than the one you have. It also has a relatively straight skirt. A fuller skirt can add a fair amount of yarn. And, no sleeves. You might find a dress on the Garn site similar to the one you want to make in construction and see if that helps.
Hope this helps.