In season 4 we see that Craster's last son is taken by a White Walker and it seems that the Night King convert him as a White Walker.
Since Craster has 19 wives at the time of the War of the Five Kings, he might have given more than one boys to white walkers.
Where are those boys now? There is no clue about child White Walkers in the series, and we can see few White Walkers with the Nights King.
Is there a possibility that there are more White Walkers (converted Craster's kids) with the Night King?