The Mini Logan is a nice bag, seems high enough quality, although it's very expensive for a child's item. The biggest drawback is the size. For a 3.5 yo, it's small enough that a child can roll it around, but it's still pretty bulky and wouldn't fit say under the seat of a commercial airline flight. Note the thickness--10"--so it's nearly a cube, as opposed to what you'd typically think of in a rollaboard bag which is thin in one dimension.
So, what you're buying is somewhere between a toy and a real bag. It's large enough that any travel you do will need to account for it. It's one of your "carry on" or "personal item" bags, and we all know how much airlines are watching these. But, it's also not big enough to pack much, i.e. to really be used like you would a carry-on suitcase. So, before purchasing, ask yourself, how will this bag factor into your collection of bags the next time you fly with your kid? What will it carry and who will carry it?
I'd love to see a version that was truly "mini"--i.e. much smaller and really just a toy for kids. Otherwise, you may as well upgrade to a larger bag that will actually be useful while traveling with kids.