Welcome to FT seafomgreen
First, I would highly recommend you read the sticky on top of the forum home page.
As for your calculations, they are about right, and you will learn from reading the sticky that using your miles (the miles you don't yet, and may not ever have...) for WT (Economy/coach) is the worst use of miles.
The wise way is to purchase a WT+ (premium economy) and upgrade to Club (Biz) for just 12,500 each leg.
I guess it depends on how much it would cost you to purchase a normal revenu ticket for the dates you want to travel, and I would start by checking that out first as you may well find the difference between that and what the taxes are going to cost you on the redemption booking are farily small.
In regards to not buying a seat for the kids, that's only going to work for your younger one because your eldest will be too old by then to qualify for that.