I don't know anything about these situations so I'm just guessing here, but how about a double-stroller for the 2 small children, with the 5yo helping you push it? Or maybe a cord that everybody has to hold onto, like I see nursery schools use for outings. Could you link the children to you somehow? Oh, would the 5yo accept the job of "chauffeur", leading the 3yo home on a wagon?
The experienced folks here will probably come up with something.