Re: Growing your own worms?
Interesting on "planting" crawlers.
Here in Vermont they are everywhere all naturally, some places better than others.
I am not a great picker (miss a lot) but I have a friend who runs a tackle shop and can pick hundreds of crawlers in an hour. Places that are good to pick are those that get mowed often and are well-fertilized. Sports fields and such, red-colored flashlights, warm, damp/wet moonlit night after 11pm works for him.
After re-reading OP I realize you are trying to do something fun for the kids- I would recommend aerating your lawn several times/year, de-thatching in the spring, and fertilizing it often as recommended by local extension office. They will probably want to see a core sample of the lawn to determine clay/sand/vegetable matter and make recommendations as to making the soil healthy and how often to water. With the soil healthy and the grass being cut often night crawlers will be happy and propagating. They will be as actively re-populating as the support structure of the soil- you will have a gazillion in a healthy lawn, not many in hard, unhealthy lawn.
Side benefit: greenest lawn in the neighborhood. Think golf course