Welcome back to the site, and congrats about the kid.
A new chain of the stock size (98 links) will work with a 12 tooth, but it will be about halfway through the adjustment range right out of the box. A stock length chain can still have a lot of life left in it with a 12 tooth when those adjustments bottom out. Then you just have to take a link out of the chain and you're good to go.
The real measure for the wear of a chain is taking a measurement of the length of the links, I think usually specs are given for how long 5 links of the chain are supposed to be, and then if it's out of those specs it should be replaced. Then of course if you have links of the chain that bind up it should be replaced as well.
Depending on how worn your chain is you really should replace the chain and both sprockets at the same time anyways. A worn chain will really accelerate wear on a new sprocket, and vice versa. As chains and sprockets wear they form a unique pattern, the shape of the teeth on the sprockets change, the links in the chain stretch, etc. As a new chain and sprocket wear they will match each other and wear evenly. But put together one that's new and one that's old, **** can hit the fan when you get up to higher speeds.