Verne says Jules was using an Etch-A-Sketch when he was 2. Jules was probably born in 1886 or 1887, and I'm pretty sure that by 1888-1889, they didn't have an Etch-A-Sketch!
Ed. note: But they're time travellin' kids! That doesn't really count. FYI, the Etch-A-Sketch was first produced by the Ohio Art Company on July 12, 1960. (link)
Response: True, they are time travelling kids, but back in 1888/9 when Jules was 2, Doc had not finished building the time train. He probably took about 10 years to build it. So, without a time machine, how could Jules get the Etch-A-Sketch?
Ed.: Once a time-traveller, always a time traveller. There's an unwritten time travel story in there somewhere, not a goof. But, I'll leave up this debate; visitors to our pages may decide for themselves...