I believe my girl was about that age when I started taking her on rides. I used a locally made/sold seat of molded plastic that fits between your legs so you have to ride bowlegged, straddling the thing. Of course that’s uncomfortable (for the adult) and harder work, but I wanted her in front of me, not behind where I couldn’t see her. That was probably a good thing, too, because she fell asleep on almost every ride and her head would fall down forward, so even though she was strapped in, I would reach around with one arm and hold her upright, pedaling one-handed back home with her.
We almost had a serious disaster once when I was riding her in that seat and suddenly there was a huge thump and the bike came to an abrupt halt. “What the. . . ?” I looked forward and saw – holy shit – she had stuck her foot into the front wheel, which spun it abruptly to the fork where it became lodged firmly, completely between the fork and the spokes. Not only that but she was wearing sandals. “Shit, shit, shit. . .” I quickly dismounted and tried to spin the wheel backwards to dislodge her, to no avail. Finally after a couple of minutes I was able to wrest her foot free. Miraculously, she wasn’t screaming, but I was petrified, certain there was very serious damage. I rode home ASAP, carried her upstairs and told my wife, “get a bucket of icewater.” No explanation, just get the damned bucket. I figured we’d stick her foot in there briefly before we went to the hospital for xrays. But miraculously the sole of her shoe had borne all of the brunt in the incident, stopping us and completely sparing her from injury. She was fine. Anyway, we got lucky. Whatever seat you get, make sure your kid can’t stick limbs in the wheels.
Despite those stories, she never did suffer an injury on the bike and she loved it.