Got the SNES mini and tbh, as cool as it is to own one, as its a nice little thing to look at, most of the games haven't aged well. There are one or two exceptions, but yeah. I can presume it'd be the same for the Sega Mega Drive Mini. I'm a pretty avid gamer though so maybe that's only heightened it.
It's not even a graphics thing - its in a gameplay way too. There are lots of modern games that are still released with a pixel/2d style like most of these had, but they're modern, so they're super responsive, varied in their challenges and have a helluva load of variety. Just better designed in general due to the industry learning loads and improving their craft. Most of the older ones are exceptionally crude when compared. Slow in terms of response, repetitive and a bit too simple. Not all of them of course, but games are largely better today, simply because the industry is just much more experienced.
Tl:dr - it'll be something you pick up, mess around with for a night or two, and then probably never go back to it again. That's what I did, despite loving all those SNES games when I grew up. Nowt wrong with that of course, but i'd say lower your expectations. As others have said, doubt your kids will give a shit really.