The watchlist isn't really a good replacement for "show unread posts since last visit". I agree that you should be able to ignore whole sections including new children, but it seems a little difficult to add that feature.
Does anyone know: If you have a board ignored and then I add a new child board under it, will you automatically ignore the child? Probably not.
Does anyone know: If you have a board ignored and then I add a new child board under it, will you automatically ignore the child? Probably not.
It doesn't automatically get a check mark in the ignored list, but it doesn't show up in the listing from the main page. If you're using "show unread posts since last visit," posts in it will still show up, but you'll be unable to navigate to that page manually.
That works well enough for me, but I don't try to drink from the firehose.
I check my watchlist, then manually browse to the boards which I want to read, but not enough to watch the whole board. 
