When my kids were little (this was back in the late 90s) they’re both in their 20s now), I purchased a used steel frame road bike with slightly bigger tires (think gravel size in today’s bikes) and mounted a child seat on it. I started taking them on rides of 15-20 miles before they could even walk. But they enjoyed it as the rush of fresh air (or maybe it was boredom) usually put them to sleep. I worked nights, and my wife worked days so we got lost of riding in. It was a rear mounted seat. At the time those front-mounted child seats weren’t widely available. It wasn’t ideal miles. But it was miles. Another key to this was riding location. When riding with the kids, I stuck to a MUP that was just a couple miles from our house. The streets/roads to it were safe, with bike lanes. I wasn’t comfortable with them on busier roads.
Tips
Make sure you have diaper changing supplies. Don’t get out of the saddle to crank uphill, for example. They kid’s head whips back and forth. Snacks and/or beverage. Give the child a few toys, but tie them to the seat or they’ll get thrown overboard. Helmet, eye, and sun protection. You might consider some sort of flat tire prevention. I did have a few flats while riding with the kid and did manage to make repairs. But obviously it’s a challenge.
Dan