Hi,
I hope you realize that there is no way to know the exact answer..... So the only way is to get a estimate number. To do it we shall have to make several assumptions.
Assuming that the present world population is 7 billion (some doubt it !). Assuming that there are much more young people than old people so le'ts say there are 4.5 billion people in the 0 to 25 group. Assuming that within that group there is an even / similar distribution than there are 4,500,000,000 / 25 children who are 7 years old..... = 180,000,000 or 180 Millions. Remember it's an estimate.
Best regards,
