This has reminded me of something, though. One thing that I like to do, for safety, is, once they have learned to sit, to teach them to sit automatically whenever you get to a curb that you'll step down off of.
So, if you are just going around the block, not a big deal. But, if you will cross a street, it would be good to stop before stepping down, and have the girls sit. Then, tell them to cont.
Since sit is not learned yet, just the act of stopping at the curb is good.
It may stop them from running into the street one day, but even if not, it's safer for everyone during walks if they aren't pulling toward the street.
If you stop and they sit, or even stand for now, at curbs, you can look for cars which aren't stopping, even if they should be, then proceed when it's safe to do so.
It's great that they are going to learn some things before the class! Have fun!
Love All Life, Thank You For Posting! :)