I'm just going to reiterate a couple of points:
Do download Caskbill's wonderful planner and play with it. You may discover that you need far more (or far fewer) points than you think.
Don't buy more points than you can afford in your budget. You will need to do two things with those points - pay maintenence on them and take vacations with them (that aren't cheap). (You can rent your points if you can't take the vacation or if the maintenence become burdensome). We DVC members hate to see people not benefitting from DVC, but I think we hate it more to see someone's DVC membership become more of a burden than a joy.
Consider your changing family. With the spread in your children's ages, even if they are the same gender, a two bedroom will be preferable to a one bedroom - and give you the flexibility to put five in a room when your eldest wants to bring a friend.