Seeing that your older son will not be with you all the time. As long as he gives you adequate notice there is an option to receive a transfer of points to cover needing a 2BR when he does come. This can be tricky and difficult as you might not find the points you need.
if you want/need a 2 bedroom for a week in april:
At AKV 2BR stnd would be 287 and savannah 372
BLT for the same time frame 2BR Standard 354, Lake 386 and magic 472 Someone else might be able to comment, but my guess is that the standards at BLT probably book first because of the lowest points.
To buy a contract with enough point to go every other year and to allow you to get a 2BR when older son is with you. you would need to buy:
AKV - 150 points ~$12750 (SSR would be in the same price point, but SSR has lower maintenance fees)
MF on AKV this would be $988 based on 2017 (SSR MF $841)
BLT - you would need ~200 points = which runs about $110 per point = $21800
MF based on 2017 $1125
So what it boils down to is -- do you plan for your typical traveling family of 5 and buy points for a 1BR (where you would only need to buy a 120pt contract for AKV/ssr) or do you spend extra buying a larger contract to have that cushion of point to cover when your son joins you. It depends on what you have for cash to buy in.
The best thing to remember is that you are not locked into your home resort unless you are looking at sept - Dec -- high demand DVC time. So if you love poly you could stay there when you don't have your son.
You do need to review the
points charts for the times of the year you will typically want to travel. If you go on davids dvc they have a good points calculator where you can put in different dates and compare all the points needed for every resort and room type. This is very helpful compared to going through pages of points charts. If you wanted to go a sunday - friday -- it would be less points than if your travel time covered the weekend (friday and saturday are higher points). There are many ways to make DVC work for your family.