When drawing a curvy chibi, there are a few places you should really look out for and make sure they don't end up too straight or boxy. On your left, we have a flat chested chibi with a curvy lower body. To do this, make sure the waist area just below where the ribs would be is indented. Then make sure your thighs come out far enough and then back in at the back of the knee. This helps create that lovely curvy feel to the lower body.
For the chibi on the right, pay attention to the breasts. They should start under the arms where the arm pits are, then create an hourglass figure by drawing the waist in and then bringing it out at the hip area. it is very important to keep at least some type of hourglass figure when drawing curvy chibis.