Mix 1 gallon vinegar from the grocery store (distilled white vinegar, for example) with a few squirts of Dawn or other dish soap (regular dish soap -- not dishwashing detergent).
Spray this with a $2.00 spray bottle on the weeds you don't like, and watch them die in 24 hours. Doing in the morning works best, because the sun will hit them all day. Do this on a sunny day, of course.
The vinegar dries out the plant, and the soap helps keep the vinegar in place until it can "do its work".
Need I point out: Don't spray this on any plants you DO like. If some gets on them, quickly rinse it off and they should be fine.
I'm sure vinegar is much cheaper than roundup, and it's non-toxic to boot! It won't harm the environment or your children.
Roundup causes cancer (leukemia, to be precise) so avoid it at all costs. It's made by Monsanto, which is the closest thing to the Devil's own corporation we have here on earth!
Matthew