There are 3 sailing dates for this itenerary in April '06. All three have different season point values (Holiday, Peak and Regular). I will try to summarize. For a family of 4 you will need at least a category 9, but (IMHO) I suggest a category 4. The points below are the high (Cat.4) and low (Cat.9) for each sailing date:
April 1 (Peak Season) Cat 9: 248 per adult, 158 for 9yr and 156 for 6yr (810).
April 1 (Peak Season) Cat 4: 401 per adult, 188 for 9yr and 186 for 6yr (1176)
April 15 (Holiday) Cat 9: 273 per adult, 158 for 9yr and 156 for 6yr (860)
April 15 (Holiday) Cat 4: 428 per adult, 188 for 9yr and 186 for 6yr (1230)
April 29 (Regular) Cat 9: 206 per adult, 106 for 9yr and 104 for 6yr (622)
April 29 (Regular) Cat 4: 329 per adult, 134 for 9yr and 132 for 6yr (924).
Having said all of that, when we go using point, we do not pay for everyone in our family with points. You can pay with points and cash. This usually results in a cash discount.
Please note that above numbers are for 2006. These rates are subject to change each year.
Sorry for being so long. Happy Sailing.