It shows as about an hour drive from East Chinaman Spring to Norris Geyser Basin. And it appears that Norris Geyser Basin is only a 20 minute drive from the intersection of Hwys 191 & 89, which they would have to have passed through to return via West Yellowstone.
The numbers I'm getting, using various times of the year, including September dates (since Google Maps won't even show that route as usable right now):
West Yellowstone to Hwys 191/89 intersection: 28-30 minutes, 14.3 miles
East Chinaman Spring to Hwys 191/89 intersection: 30-35 minutes, 17 miles
Norris Geyser Basin to Hwys 191/89 intersection: 26-28 minutes, 4 miles
So, imo, it's doable during a day trip. Especially if they told the kids "there was a really cool place they wanted them to see, where you could get off the path..."