I.B. Swimmies length will land anywhere between a little above the knee to mid-thigh-ish (depending on the length of the child).
My sizes are true-to-size based on the child's age. What this means is the average sized kid--about 50% on the growth charts--is used as the barometer for waist sizing.
If you have a child who's age fits evenly between the ranges, or you know is larger than average, it might be best to size up rather than to size down.