People always have their opinions about riding kids on bikes....if the opinion is never do it...they are WRONG !!!
Great for parents and grandparents to get that kind of time together. I take all of my nephews for rides as often as possible. I keep it to moderate speeds and on roads and areas that are a bit safer, i.e. no interstates, lane splitting, heavy traffic city stuff or dangerous mountain roads. Always in good weather and geared up....everything in life is a risk. Sending our kids to school is a risk (sandy hook, columbine, dozens more) getting in a car....some of my daughters and my best memories are out for rides on my old Harley. We would ride out to Cook's corner or Hell's Kitchen and order a pitcher of Beer {root that is} and a plate of chili cheese fries and shoot a couple rounds of pool like a couple of old bikers...great times.
I had the throttle of a real motorcycle with clutch and gears handed to me at 5-1/2 years old and I would bet it was safer on the back of Gramps CB450 or dads CB750 than twisting the grip off of my own JT60, yet 10s of thousands of people put a bike under little kids everyday. Anyone said "AWWW" when Chad Reed came out at A1 SX with his 3 yr old in matching gear and he rode away on his little yami 50?
Be careful, choose well, ride smart, gear up...but take your kids/grand kids for rides !!! They will remember it forever fondly.