Should voter ballot initiatives be allowed to overturn laws passed by legislative bodies

Personally, i believe it shouldn't, unless the number of votes required is more than half of the total population.
If, a legislation could be chosen by voters ballot, there is simply no need for the legislative branch to exist, because their decision could be easily overrun by the ballots. 
The second problem is, voters ballot will be chosen purely by voter's selfish personal desire without having to pay attention what the whole nation needed

To expose the horrors of child labor