The approximate rule
Here is a way to guess: add the heights (inches) of father and mother, divide in two, then add 2 inches for boy and subtract 2 inches for girl.
For example: if dad = 71in, mom = 65in, then (71+65)/2=68; then 68+2 =70 for the boy and 68-2=66 for the girl.
Just remember, this is an approximate. Life circumstances, nutritions, illness, long-term med use etc..Can affect it.
Have fun and good luck.