I believe the number of hours you can pre-book varies based on length of cruise -- with longer cruises allowing more hours, shorter cruises fewer hours. The intent is to make sure everyone has access to some hours in advance. Once onboard you may book more depending on availability. Based on reports, it seems that isn't usually much of a problem. Though as PP mentions if you want dinner hour every night, you may need to pre-book that instead of expecting to get it onboard.
The hours should be per-child, assuming you have more than 1 who is nursery age. You do not need to book Club/Lab time for kids age 3+.