We’ve got one of these: Safe-T-Seat. Your kid needs to be good at sitting up unsupported for a long time and able to hold their head steady over bumps (with a helmet on too), so probably not younger than a year or so.
Pros:
Works well, my two-year old loves it. Very good for being able to talk to him whilst riding and he can see where he’s going.
Nice and safe: no way for little legs to get snagged on stuff, lots of support for back, big plastic bar to hold onto, and straps to hold ’em in.
Very stable once you get going.
Easy to fit, 2 mins with an allen key to fix/remove the whole thing once you’re used to it.
Cons:
It’s a bit bulky so if you’re on the small side you might have problems riding with it on. I’m 5’10” and it’s fine for me on a pretty short bike.
Tight fit with flat stem/bars, we have to take the boy’s shoes off to fit his legs under.
Make sure your stem bolts are nice and snug or it might loosen your headset up.