For each month you have of Gold, you get $5 off of the price of Family Gold ($100), up to 20 months, because 20x5 = 100. Any time you have after that IS LOST. Gone forever. So don't add time to make your total more then 20 months.
Now, once you are Family Gold, you can add other accounts, up to 3, to the Family. For each 2 months of gold on any of those accounts, you get a 1 month extension to Family Gold. So if you add a 12 month card to your kids account given them 12 months, then when you add them to the "Family" you'll get 6 more months of time. The most time you can have is 72 months.
One little snafu. At least when I signed up, I already had my kids as sub accounts (just not Family Gold sub accounts). When I converted to Family Gold I had 1 year on each of their accounts. So then they were in my "Family", but they were NOT part of the Family Gold family! I removed them from my family and then... well, then I discovered that I couldn't add them to the Family Gold family on the xbox because the Family Center wasn't there!! I called Xbox Support at that time and they told me it was due to an "emergency problem" with Family Gold accounts and that in a few hours it would be back. I mention all that, because perhaps it was suppose to have incorporated them automatically and it didn't because of this issue. I don't know. If for some reason your existing child accounts don't automatically end up getting added, just remove them from the Family and then add them back. If you don't have child accounts already this won't be an issue either way.