T.S.
BMI compares weight to height which is why it's used to determine whether a person is over/under weight. It isn't actually done using pounds and inches, it's done with kilograms and meters hence the kg/m2 after the number... but don't stress that's just what's used in the formula... the BMI you've always heard of is ALWAYS done the same way... the calculators just make the conversion and don't mention it. The % is the percentile so ti tells what percent of children that age are BELOW the BMI of your child. Z values from -1 to 1 are within the normal range. 0 is totally average. Positive z values mean your child is above the average BMI negative they are below.
So your first daughter has a BMI of 23.47. Relative to her height she's heavier than 84% of girls her age. Z=0.98 she is above the average close to the highest end of the normal spectrum. Whether or not that is TOO high to be healthy is a question for her doctor, but you can also check a healthy BMI chart to see where the healthy range is.
Second daughter has a BMI of 19.85. That is EXACTLY average for a 10 year old girl. 50% of girls are higher, 50% are lower. Z=0 she is right in the middle.
Your son has a BMI of 15.93. Compared to his height he's heavier than 66% of 5 year old boys. Z=0.40 he is above the average, but within the middle half of kids. Again, your doctor would be the one to tell you if he's in the healthy range.
Honestly your oldest child MIGHT be slightly over weight, but she may NOT be. This is something to discuss with the doctor. For an adult, a BMI of 23 would be toward the high end of healthy (18.5-24.9), but for children there is not a hard and fast healthy BMI range because that changes according to their age.
Call the doctor and ask her to discuss it with your daughter. You do NOT want her to stress about this! If it needs to be addressed, help her take care of it. If not, help her let it go.
Hope this helps.
T.