If your dad is 5ft 11 and your mom is 5ft 5 how tall will the children be?

Generally. Mom + dad +2 (for boys) and -2 (for girls) divided by 2 then =/- 4 inches so 5-11 =5-5 +2/2= 5ft 9 +/_ 4 inches (5-5 to 6-1) 5_11 + 5_5 - 2/2= 5 ft 7 +/- 4 inches (5-3 to 5-11).
Simple estimate. Add 2.5 inches to the average parental height for boys. Subtract 2.5 inches from the average parental height for girls. 71+65=136 divide by 2= 68. For girl 68-2.5=65.5, boy 68+2.5=70.5. Some variation in final height is expected but this gives a simple target.