Are you anywhere near Holmes Run? Super-accessible moving water full of pumpkinseeds and bluegills in all of the bend pools, but most prolifically just upstream from the second trail crossing (from Lake Barcroft). Also you'll find an occasional bass. And maybe maybe maybe up at the head a holdover stocked trout that's been hanging out in the deep pool by the big boulder above the Columbia Pike bridge (I saw one in its last throes of life last August).
It's extremely easy, shallow wading, and if the kids are outfitted with polarized shades it's just sight-casting. Hang out until you start to see the bluegills school, then just toss flies over them from 8 feet away -- no need for a back cast, more just like a dip -- might as well use a 6' stick with a bit of tippet on the end. These are hungry, relatively unpressured fish but they are somewhat picky: I would tie a different pattern onto each kid's line (2 dries and a nymph perhaps) and see what's working, then switch everybody over to proven patterns once you start to get strikes.