Children normally require 2 hours per week for 10 weeks preparing for the tests. This would be sufficient for them to reach their potential. Any amount of practice, either at home or with a tutor, will not ensure that a child will pass the selection test if they do not have the potential. Whether they practice at home or with a tutor depends on how confident you feel about helping. It also depends on whether, from your previous experience helping with school work, your child is comfortable with you in this situation.
Bear in mind that tutors do vary - some are very good - others would be useless - and it can be expensive. If you do decide to employ a tutor the best way to select a tutor is by recommendation from a friend. If this is not possible try Yellow Pages or adverts in the local newspapers. Ask how much experience they have and what the total cost will be. Also what material they use - many do use the Secondary Selection Portfolio.