Angelo,
Rob Snowhite's prowess on 4-mile is widely known. He should be able to help you.
As for your son, bobbers with typical sunfish bait have worked well with kid fishing events held at the bridge crossing over a small feeder tributary to 4MR. You'd be standing on riprap reinforcing the bridge foundation. There is usually a population of bluegills, sunfish at the mouth of the tributary. The tributary is a short walk upstream from the parking at the end of Commonwealth Ave in Alexandria.
Hope this helps.