Do you know how to construct truth tables?
Here is one for three variables:
Now replace F with B for boy and T with G for girl.
Then you show how a family can have three children.
If I understand your question correctly, you are asking how to conduct a simulation in which you have to make a random choice between two alternatives, each of which is equally likely, without flipping a coin.
There are many ways. In computer simulations, one generally uses a "pseudo-random number generator". This is an algorithm which produces numbers which look like they come from a Uniform(0,1) distribution (although they actually do not). You could draw a random number from the generator and say the child is a boy if the number is less than 1/2, otherwise a girl.
Alternatively, you can use some other physical method of randomization; for example, you could roll a die and say the child is a boy if the number on the die is even.